No, it's not just any assumption, there's the fact that Burke is handing out free tuition for the girls in Quinn's prostitution ring, something he was going to do for Maya until she mentions that she was actually considering joining the HOTs, at which point Burke immediately changes his mind, and tells her to quite college instead, he's providing them with free tuition in exchange for sex, it's completely obvious. And after his discussion with Maya he needs to talk to "Priscilla" (Quinn):
He can't give Maya the free tuition since it would interfere with his deal with Quinn and the hookers.
And there is at no point ever hinted in the game that Quinn has had sexual relations with any guys except Vinny and MC, the only objection you have to this is that it doesn't fit your image of Quinn.