Well, after having been to uni and going clubbing alot, this is what I've witnessed. Now, I will say it tends to happen, not that it happens all the time and certainly not that all guys do it. A joke I heard on the subject pretty much covers it:
"You'll notice early on most of the conventionally attractive girls are taken or being hit on. As the night drags on and the club filters out, it starts to become much like clearing on UCAS. Guys just end up going for whoever they can find that isn't taken."