Ugh, same. Back when I was married both in spirit and law, there was a nice, well-endowed bartender at my local. I had no designs on anything at all, wanted to just have regular ol' barfly-bartender platonic chatter. Here was my inner monologue:

Don't stare at her boobs, don't stare at her boobs, oh god I'm looking at her boobs, look UP you fucking creep she doesn't need that shit; ok, maybe just identify all the liquor bottles behind her - THAT'S A FUCKING BOOB KNOCK IT OFF - k, Wisers, Captain Morgan's, some coffee liqueur - wait, she said something. What did she say? Oh shit, she's got the customer service scowl on and has covered herself. Congrats, Cracks, you are now officially one of the creepy guys, no better than the alkie dude who asks for hugs. God damn it - go play some pinball and go home.

Stupid lizard brain. Doesn't happen all the time with every woman (thank fuck, that would be paralyzing), but awkward as fuck for everyone involved when it does.