I would say yes. Basically, there's always going to be a heirarchy at school, which was probably based on brute strength rather than anything else.
So, if you randomly meet someone in a bar who you last saw in your class some years ago, and this guy...
a) used to be a big cheese at school
b) is someone you didn't really like that much
c) has a fuckable wife/gf/partner
d) suddenly isn't so much above you these days
...then I'd say the woman in question is fair game.
Mind you, having said all that, I would probably unhesitatingly fuck the wife/gf/partner of my best friend...