For guys, who are filled with insecurities, being with a drop dead beautiful woman is very intimidating. Us guys all know that she can have her pick of any guy that she wants and while she currently wants the guy she is with, she inundated with gorgeous guy every day. We don't know why she is with the not so spectacular guy now but figure, given a womans right to change her mind, that she could change at anytime. Like fashion. Very few guys are confident enough to know that she will be with him forever, or at least the end of the week.
But if he finds a plainer, and maybe slightly less intelligent woman, she will know how lucky she is to have the guy and won't be sneaking around screwing every tom, dick that she sees.
But that doesn't mean that he would not like to be screwing all of the beautiful women that he can. They may not stay long term, but tonight is long enough.
And finally, guys are like dogs. A yes means forever, a no means not now, but maybe later. So he knows that you are currently saying no. But bo use in burning a bridge, you might feel like getting naked tomorrow. He just wants to keep his hand, relatively speaking, in.
Almost all guys are filled with insecurities. Am I tall enough, good enough body, smart enough, big enough, funny enough, sensitive enough, etc.