A funny thought occurred to me recently:
What if those medieval landowners didn't take the serf's bride out of pure lust, but rather as a way to spread their "regal genes" ?
There is no evidence that the droit du seigneur existed, here is a post going more into depth on why it is a myth.
https://www.reddit.com/r/AskHistorians/comments/3e2p7s/do_we_have_any_evidence_that_droit_du/