They need human sperm to exist in our universe.
Three universes, one on either side of our own. The beings from outside need sentient sperm to survive in our universe. Yes, there are angels, devils and monsters. It seems humans got it all wrong, except for the monsters. It sucks when you're no longer at the top of the food chain.
When "they" need human sperm to exist in our universe you can be sure "they" will do whatever they have to do in order to get it.
Aliens and humans interact on alien terms in the hope both survive.