Let's start with the physical challenges. First of all, being in space lowers your blood pressure and slows down the flow of blood overall. That means you almost certainly can't get a boner. So there's that.
"I'll take that challenge."
If you somehow are able to achieve a zero-G erection, you'd better damned well make sure you've got multiple forms of birth control in effect. Think about it; they're not going to stock an interstellar mission with tens of millions of dollars of extra supplies on the off chance an extra crew member will pop out of somebody's womb. But that's not the only reason a pregnancy in space would be bad news.
Experiments done on mice embryos have shown that a lack of gravity messes with an egg's ability to properly develop. Add to that the markedly higher doses of radiation an astronaut is typically exposed to, and the result is not going to be something whose picture you'd want to keep in your wallet.