because that is the sense I got from Lydia's letter to Harriet. She, Lydia, was under the impression (whether Wickham told her or alluded to it is obviously unclear) that Wickham wants to marry her.

Having said that, I find myself in agreement with those who characterize Wickham's short term tendencies. He is a philanderer, and an opportunist (whether the opportunity is to bed someone, or to marry for money is irrelevant apparently). In this case, Lydia was a willing partner and a very tenacious one at that. He probably didn't care for Lydia, but may have implied some things to get what he wanted from her...