The sex issue is just frivolous. E.g. I haven't had anything but rejection from a woman in three years, I'm perfectly healthy and functional, and I'm not being carefully chosen for one of the most groundbreaking missions for the human race.
Monks can train themselves to be celibate, where sexual thoughts and feelings become superfluous and uninteresting, so why not astronauts? It's all about discipline.
And there's always the option of having plenty of sex before going up and self-relief when you need it. If pleasure is more important to someone than advancing humanity, then by all means stay on the ground.
Really, this is not an issue at all.