I'd hit that.
Levite explains another aspect well: we're all big kids, and we have fun acting as such from time to time. High school, for me, was one big "I'd hit that", and so was part of college.
Not only is there the juvenile appreciation of the female form, but there is something to be said for unbridled truth. I mean, I really would hit that. I really, really would. And instead of turning on all of the filters that say "You can't just say you want to shag a woman outloud" we simply say exactly what we're thinking.