He was just as interested in sex with Picard. And you are correct, it's not how we would see it. He picks the top human on a ship and tries to make them like him. He eventually figures out that sex is the ultimate expression of liking and so if that is part of the wooing success he's up for it.
His ultimate goal is to have the human be wooed.
I still don't see why he would want to have sex with Janeway in particular. Q seemed like a wacky neighbor from a bad sit com who hits on on the female protagonist, instead of the character he was in TNG. He simply wants to produce an heir, he could have done anything, he could have appeared before some college girls in the guise of Robert Pattison or he could have used his powers to create the perfect bride. There is no good reason why Q would spend all that time trying to woo Janeway, when all he wants is a to have a child. He could that with almost any woman.
This episode just plain sucks, it contradicts almost everything we know about the Q and makes them far less interesting in the process: