Namely: astronauts try NOT to as much as they can, and when they do go, it's a mess for both them and their crew mates. They suffer through it because being in space is a worthy achievement.

Apparently it's such a mess that NASA estimates this is why astronauts tend to undereat. Apparently Gemini 7's Frank Borman spent 9 days without going number 2 because of this, and planned to hold it in 2 full weeks (the article doesn't clarify whether he managed). Skylab seems to have done some progress, but we're still in the early eras of space toiletry!