Chris leaves Africa with Jill on one arm and Sheva on the other. Now he's surrounded by dudes. Wesker had that hot Italian chick in 5, but he incubated a monster in her. Leon has worked with Claire twice, and she clearly adores him. He was directly propositioned for sex by Ashley, and by all rights, he definitely earned that lay. Now he's working with Helena to no avail, still holding a torch for the woman who constantly betrays and manipulates him.
Who's to say Chris didn't get some action after that? Same for Leon. Maybe they're just the hit and quit types. I dunno.. I think you should real aim your question at the writers who control what the characters do and say.