Yes, though I was (mildly) hoping you wouldn't ask, since it's a nuanced issue; I do have some time tonight though, so here it goes.

I'm going to give you a tl;dr right up front, but I expect you to read the full argument if you have a problem with the summary:

1. Animals can't be held to the same standards as humans (or future humans, i.e. children), since they cannot and cannot ever buy into our ethical system; however we still have an ethical obligation to them, since they, being sentient and emotional creatures, have an innate right to welfare and some limited autonomy.
2. Because they can't buy into the ethical system, they don't (in fact aren't capable of) caring about what your intentions are; intentions only matter to those who buy into an ethical system, since the very notion of intention used in the context of action presupposes a framework where some intentions are different (better or worse) than others. This is ethics by definition. The animal is only capable of valuing the result of the action (the action itself, not the intention).
3. An animal either does or does not value sexual advances; the animal is, if reasonably healthy, capable of demonstrating this (through accepting or overtly rejecting said advances) -> see National Geographic reruns or your university animal behaviorist if you need evidence of this.
4. At this point, there are several possibilities: either 1. you allow that animals can value sexual advances, they have some form of limited autonomy, and therefore it is not unethical to perform/engage in said advances when the animal is receptive to them, 2. you deny that animals value any sexual advances, in which case (since animals can't value your intentions), the act of breeding animals, reproductive examinations, and any other such sexual action is now unethical by your framework, or 3. you believe in a contradiction, since you simultaneously allow sexual action with respect to the animal (with or without its valuation of that) in the form of reproductive exams, breeding, etc. while denying sexual action in the form of that which involves pleasure. This is a logical fallacy. Your alternative at this point is to argue that its unethical to commit sexual advances on an animal who is receptive to them due to social norms or the ethical system in place. However, in order for something to be unethical, it must be in direct violation of a system of ethics; such an action is not in violation of a system of ethics, because it does not infringe upon the autonomy/welfare of others, poses no threat to the public good, and otherwise has nothing at all to do with anyone aside from the human actor and the animal receiving the action.

Full argument: Proposition 1: An animal cannot reciprocate in an ethical system, therefore the autonomy of the animal must be limited.

Assumptions:

1. All sentient beings who are alive value (implicitly or perhaps explicitly) their autonomy and welfare
2. It is always an ethical negative to violate the autonomy or welfare of an individual
3. Welfare and autonomy can be reasonably violated in circumstances where it protects the welfare and autonomy of many others, such as for the public good, or in the case where violation of an individual's autonomy and welfare in the short term results in a greater benefit to that individual in the long term (e.g. medicine).
4. Violation of an individual's autonomy or welfare is otherwise an unethical action
5. Unethical actions should be avoided or prohibited

Lemma: An individual, in order to be a part of an ethical system, must be capable of adhering to that ethical system.

An ethical system is, by definition, a system designed to promote ethics. By the assumptions, an element of any ethical system is the respect of autonomy and welfare of individuals. A fully autonomous actor is capable of committing any action; in order for any action committed by the actor to be ethical, it must adhere to the ethical system. However, if said individual cannot (mentally, emotionally, or otherwise) understand and therefore avoid unethical action, then the actions he performs will include unethical actions. Such an actor is not compatible with an ethical system. Therefore, an actor who belongs to an ethical system must be capable of adhering to the ethical system.

Lemma: An individual who performs unethical actions must forfeit some of their autonomy, willing or otherwise, in order for the ethical system to thrive.

A fully autonomous actor can commit any action, ethical or otherwise. Unethical actions are not compatible with an ethical system. By definition, any enforcement of unwilling action on an actor violates his autonomy. In order to prevent unethical actions from an actor, his autonomy must be violated, at least to some degree.

Suppose: An animal, insofar as the animal is a sentient and emotional being, has an inherent right to welfare and autonomy.

The cannot hold for the animal being a fully autonomous actor; the animal is incapable of participating in the ethical system, therefore the animal's actions will include the unethical in the context of the ethical system. The question is then: how much autonomy should the animal have? By previous arguments, an animal who cannot buy into an ethical system must have its autonomy limited partially. In current practice, this includes ownership, though it could involve scenarios like wild-life protected areas, etc.

Proposition 2: The intent of an action, given an actor, only has ethical relevance to individuals who are part of an ethical system.

Assumptions:

1. Individuals who are not part of an ethical system are either incapable of it or have chosen to commit unethical actions and are therefore incompatible with it.

Consider the second case where a fully autonomous actor commits unethical actions. By proposition 1, his autonomy must be limited. This is how the justice system operates; actors who violate an ethical system are deemed incompatible with the system and their autonomy is limited until they are deemed capable of participating again. While they are incarcerated, the individual whose autonomy is limited by definition is outside of the ethical system and has no ethical obligation to those within it; if he had an obligation, then this would be a contradiction of the premise of an ethical system, which is that autonomy cannot be violated except for the case of the greater public good. Since the individual with limited autonomy is in prison, he poses no threat to the greater public good, and therefore limitation of his autonomy while simultaneously expecting compliance with the entire ethical system is a contradiction.

Other actors still within the system still have an obligation to the individual not in the system, since part of the ethical system is to respect the welfare and autonomy of individuals (rights to which the prisoner still has in full and in part, respectively). The intent of actions committed with respect to the limited individual matter, but because the limited individual is not part of the ethical system, only the result of action on him is significant to him in an ethical sense, since intention presupposes ethics, and he does not belong to an ethical system. The intention matters to the actor still within the system, since that actor still buys into the ethical framework.

Consider the first case, where individual removed from the system is incapable of participating in the ethical system. Here, the intention of the action against him is not only disregarded, it is irrelevant, since the individual removed from the system is incapable of understanding the actors's intent; he can only understand the result of the action itself. The intention behind the action still matters to the fully autonomous actor, since he is a part of an ethical framework, and the intention behind his action can be perceived as better or worse in that framework, by definition of ethics.

In this case, the individual with limited autonomy, be that a human child (though temporary) or an animal, can only perceive and value the action with respect to him in terms that he understands. In the case of small children, this is either the action itself or some limited perception of the motivation behind the action. However, when that child grows, the action's intention will matter to him, since we have memory. For the animal, the only capacity of valuation he has is for the action itself; no animal (not of human-level intelligence) can or will ever value the intention behind an action, because the animal is incapable of doing so.

Proposition 3: All sexual actions with respect to the animal are either ethical or unethical.

By definition, a sexual action is one involving the reproduction, the reproductive organs, or engagement in direct reproductive behaviors or actions with another individual. This includes animal breeding (artificial or otherwise), reproductive exams, hormonal stimulation, sex, etc.

Given the above arguments, the animal is either receptive or not receptive (he either values or does not value) sexual advances. If you grant the animal welfare and limited autonomy (as is required of you if you are an ethical human), then you either 1. acknowledge that the animal is capable of such valuation (values sex), in which case there is no unethical aspect to sexual action with respect to animals, or 2. you argue that the animals are incapable of valuing sex, in which case all sexual actions with respect to the animal are therefore unethical. Any such position where breeding/reproductive exams, etc. is ethical and other sexual actions are not is a logical contradiction.

The corollary to this, which is obvious if you work with animals on a regular basis, understand that humans are animal too, or have any familiarity with animal behavior, is that animals do indeed value sex (as do humans). The conclusion follows from there.