The following is of course subjective opinion, but since I like to disassociate from some of the biological impulses programmed into my brain, I'll pretend I'm describing mankind in general. I don't mean to speak as a representative of all men.
The heterosexual man who is not in-love with a specific woman has a general affection for all women. It is like he is in-love with womankind. This is a happy and pleasant time, for a generalised, abstract notion of woman doesn't demand anything from him. Yet all the wonderful possibilities remain valid.
But when one particular woman gets his attention, and it seems there may be a possibility (however remote) that he might have her, then begins an excruciating logistical process. This logistical process is partly conscious, but mostly subconscious, and is something like a number crunching exercise that determines whether the woman is ultimately worth the effort. For example, is the man willing to trade his friends, his hobbies, and life as he knows and enjoys it, for the sake of this woman? Is he willing to risk a broken heart, raging jealousy and suicidal despair?
There would be no mental gymnastics required if the man got the woman his heart truly desired. When a man loves a woman, she is the sum of his desire. And if he were the sum of her desire, he would endure eternal hell fire for a minute in her arms. But subconsciously he knows this not to be the case. Subconsciously he knows that to the woman, he will only ever be a means to an end - and that end is ultimately offspring.
Therefore, in determining whether a woman is worth it or not, he's determining whether a momentary illusion of having the woman is worth the inevitable strife that will result. Not to mention other criteria such as whether he has a hope of attracting the woman in the first place, and once he's got her, how long his wealth and influence will be able to hold on to her.
If the man receives a green light from his subconscious calculations, then begins an intoxicating time where the object of his love has no flaws. She becomes transfigured, and to be near her is to be alive. Even the mundane moments of life take on a special glow. The sniff of dung in the air becomes like a sweet perfume. If he manages to woo the object of his love, these feelings intensify. Then comes sex, and that first ejaculation is the ultimate triumph.
It's interesting to note that if he does not woo his love, or if the calculations are not favourable and he gets the red light, she will become less than ordinary in his sight. Her flaws and irritating traits will become magnified, and he will thank his fate that he did not get sucked into her tar pit trap. This isn't always the case, but for the vast majority it is.
What he is looking for in an ideal mate is of course health. He may not recognise this consciously, but there is a very practical aspect to his admiration nevertheless. Does she have nice ample breasts to suckle young? Does she have wide, childbearing hips? Is her vagina clean and free from infection? Is her skin in good condition, and does she have an adequate fat deposit to sustain a pregnancy? If all of this is true, it must mean she possesses mental health. But even if she has a less than ideal figure, is she kind? Will she care for her children and her man? Is she promiscuous? Is she greedy? Does she possess other negative traits? If so, well she might only be good to impregnate and abandon. Otherwise, she's worth fighting for.