I remember reading reports from the early 1980s about the psychological aspects of long term isolation. Scientists were studying what happened to the "wintering over" crews in Antartica to get insight on what would happen on a Mars mission. The report described what happened when a woman was first added to the wintering over crew. The crew started experiencing hostility towards one another and there was even a murder. The woman also was very isolated socially. Things improved somewhat when they added a second woman to the crew.

I spent a year on an isolated island in the Aleutians. There were several hundred of us there, perhaps 40 or so were women. Lucky thing for me, I was (and still am) a happily married man. Among the others, there was a lot of competitive behavior between the men as they pursued the women. Some women liked it but a lot didn't. Shortly before I returned home, we had a new female airman join our squadron. She was easily the most attractive woman on the island. There were guys hanging outside her door all trying to hook up with her. She was finding the experience overwhelming.

Admittedly, the people selected for a Mars mission won't be average in any way. They'll be highly educated, disciplined, and carefully screened both physically and psychologically. Still, primal urges in confined spaces over long periods can't easily be dismissed out of hand. Those wintering over crews were carefully screened, too.