Hmmm, I'd say pretty normal. Male guppies can be pretty persistent!
Although I don't think she could get pregnant...since they are from different families.
Nothing can realy come out of the mating between the male guppy and molly but it sounds like he is a bit sex-starved to me; guppys have very high sex drives and will try and mate with other species of fish if there are no females in the tank so it might be a nice idea to buy him 2+ females to socialise with .
Yep, I was just about to say what Toxis-Phoenix said!
Getting him a few females would be good. Just make sure you get more then one, so he can divide his attention. You don't want one female getting stressed!