How do astronauts on the ISS relieve their more...personal needs?

I recently got the game "Space Station Sim", I've read the book "Gravity" by Tess Gerritsen, and I've generally had an interest in the International Space Station for many years. I've also seen the movie "40 Days and 40 Nights". Now, the ISS has the unique problems of staying in space for extended periods. Also, I may be wrong, but I don't think the first astonauts and cosmonauts were women, and I thought men probably had to stay up there for months at a time. And even if there are women, they probably wouldn't always have sex with each other anyway. So then, what do the men on the ISS do? Do they remain celibate for months on end, from the day they go up into the ISS to the day they return to earth, or is there some way that they can relive their tension (masturbate) without getting messy. If so, do they use a special procedure, or is the same system they use to urinate, and for lack of a better word I know of, poop.How do astronauts on the ISS relieve their more...personal needs?
A few months is nothing. Just keep yourself busy with your work and everything is fine