There are two reasons, though probably more, that I see it making it difficult to extract Kant's view of homosexuality by his philosophical system from the views of his time:

First, Kant's philosophical system just doesn't have a place that I'm aware of that sexual identity in general, be it homosexuality or heterosexuality. I believe "homosexual" enters language in the late 19th Century but even the whole frame of sexual identity would be, I think, just straight up alien to Kant.

So, rather, we might look to something like homosexual acts, or something like that. Typically, arguments against the moral permissibility of homosexual acts ride on a view of natural teleology, such that homosexual acts fall into a category of perversion of the natural function of sex organs, along with non-procreative heterosexual acts. However, I'm not all clear on Kant's view on natural purposes, and imagine a strong view of that with respect to humans would quickly run up against Kant's ethics of people being ends in themselves, rational beings as legislators in a kingdom of ends, etc. I'm not the one to say anything further in that respect.

(I'll have to edit this later when I get a chance but the point will be on the 'objectify' natural of sexual desire which, I think, is potentially more radically restrictive by its own logic that Kant's contemporaneous thoughts on sex)