The daughter of some very powerful NPC falls hopelessly desperatly in love with him and daddy comes by to make sure that he 'does right by his little girl' (Who is, of course, pregnant).
Or his reputation could begin to precede him, he may be a notorious lover to the ladies but men, obviously have a lower opinion of him.
Or combine both of these. He starts to leave behind pregant women. His reputation precedes him. Everywhere the party goes, paternity suits await them (even places they have never been before -- there are always young pregnant women who need a scapegoat so their true love isn't killed).