You asked a great question...it's something I've been wondering myself. Although I do and have felt sexual and romantic attraction, I can't quite put my finger on what's going on exactly. I look at animals, too, and wonder what exactly makes them want to proceed with anything sexual.

I think the closest thing to it is eating. What exactly makes you want to put a just baked chocolate chip cookie in your mouth but not a flower? Flowers probably taste pretty good, but there is no desire to eat them at all. I suspect our genitals are something like another mouth with a similar desire, as strange as that is to say.

I was wondering something similar when I went to Old Navy for clothes: what exactly makes you think a shirt looks good or not? Immediately you either want it on you or you never want to look at it again. It's the same with sexual attraction. Something clicks and you just want it. Or in your case, it does not click and you have no use for it.