OK, I'm trans (duh?) and bisexual. So, this doesn't come from a cishet girl, and it might not be the perspective you're looking for. I am incidentally married to a ciswoman, but attracted to certain guys nonetheless. (So most of you can stop reading now. LOL)
I become attracted to guys when I just click with them, share interests or a sense of humor about things. Being a sweetheart doesn't hurt either. Looks can fade over time, hair can thin out, beer bellies can swell, but the core remains. Having a "Y" chromosome is not on the list of characteristics I'm looking for. At the end of the day it's not important. I am in it for the long haul, and not at all into the himbo-du-jour like some of my friends.