You know what I have always thought. Why doesn't the victim just run? the rapers don't really try to hold them down. It's more like the victims just let them because they are so scared.
Maybe the dominator uses it's weight to pin down the partner while reproducing, meanwhile the dominated remains in please unable to hop or slide away out of the dominator's weight pinning it down.
That or somehow, it's "game over" when the peni-peni get's inside and they are locked in some way that can be unlocked easily, i do recall a comic or 2 about yukkuris reproducing but some accident or something didn't let them deattach and remained unstuck...