Elon Musk doesn't want to simply send humans to Mars. The SpaceX CEO has bigger ambitions. He wants us to be an "interplanetary species," which means creating a self-sustaining civilization on Mars, which means living and dying on Mars — which at some point might mean sex and pregnancy on Mars.
So how would that work?
Given that Musk hasn't figured out how to keep people alive on the trip to the Red Planet, it's unlikely he has details on how people will make more people once they're there. We don't have any data on how human bodies will work on Mars specifically, but we have enough information to know that sex in space could be a real hassle.