Since each match will end with fucking, what differentiates 'winners' from 'losers' other than maybe who's on top? The scenario could easily feature explicitly bisexual mixed-couples matches, either 1-on-1's, or tag-teams, or free-for-all. Lone winner royally fucks all losers.
Jim and Josie face off against Ken and Kara. After Jim takes-down Kara and Ken 'beats' Josie, it's just Jim and Ken. Winner gets the women again. If they tie, do they get in a couple of DPs? Or the women trounce the men in the first round, cowgirl-riding their cocks triumphantly, then confront each other. Whoever wins will enjoy much clitoral stimulation. Or all four are in the same ring. Josie quickly subdues Ken, takes a quick reward fuck, then joins with Jim to overwhelm Kara. Now it's Josie vs Jim -- who cheats? Or maybe Josie and Kara together thump Jim. Two reward fucks, then on to each other.
No, there are no rules, but if an opponent is disabled, they won't be much of a fuck, so it's in the fuck-fighters interest to not cause much damage.