Humans were so naive sometimes. They hadn't even managed to find life on a new planet yet. But the male aliens had known about them for years now. They'd been visiting and taking males for years. They needed male humans to carry their offspring and mate with. Their species had no females and it had long ago being discovered that male humans DNA could merge with theirs to create a life.
Over the decades of experimentation, and numerous male deaths while trying to get them to carry a baby it had recently been found the problem. The males needed a womb. And that's exactly what they created for their mates so they could implant them and they could repopulate their dying population.
Of course, their mates were used for other things other than pregnancy, only carrying a child for part of the year, and it was their main function, but the aliens loved to have annual games in which all mates would compete for their masters glory. Trophies for the masters, piercings for the humans. Aliens loved it, humans were a little more... worried, though some of the manly ones were all about the winning now, and had adapted to their new lives.
So that's my rough idea. I'm back from my haitus now, so let me know if you want to work with me and we'll get cooking up some rough ideas :) I am also looking for someone to play the alien.