I agree with what Tracy W says. Wickham makes poor, short-term decisions, based on immediate gratification. He wants to have sex with Lydia and it is very clear that he won't have much trouble succeeding, especially if he gives her a line - how many men do this, telling a girl they love her just to get her into bed?? It's a timeless habit, and one that often succeeds even today. He could have told her he loved her, even if he didn't specifically ask her to marry him - alluding to it would be enough for her. (Remember that Willoughby never proposed to Marianne in S&S either, but she assumed there was more to their relationship - excuse the reference to another novel).