Hit it and quit it.
But be nice about it.
I'd assume that's a given, coming from me.
I spent the last seven hours talking to a guy who is 3x divorced, and is now engaged to be married again.
Never once did we say "hit it".
Lets not forget I'm an asshole.
99.9% of guys are assholes to some degree, so that's not really much of qualifier.
It's more of a youth thing, not necessarily "younger generation" - just younger. It's indicative of how young men think. I've got a couple of friends in their late 20's, early 30's range and you can occasionally catch them snickering when we use these type of qualifiers, because to them, it's somewhat juvenile. They've found more eloquent and flattering ways to avoid being seen as "a pussy" in the eyes other other men, or even have the maturity to just no longer consider that a motivation on their actions and speech.
This is one of the few areas in which I differ online to reality. I would never seriously say "I'd hit it" about a real girl I was interested in, maybe because most of my friends are female I've just developed better ways around it. In fact, I generally flippantly dismiss the questioning; "Sure, she's attractive, but I don't know her, man." Generally accompanied, by an offer of drinks and a little anecdote.
I digress; what I'm saying, is that using that little phrase is more indicative of a guys mental maturity and experience than as a genuine "asshole" qualifier. If a girl finds the phrase unpleasant, it probably means she's interested in a higher caliber of guy.