A close friend of mine was on a previous Hi-SEAS, and I can say that this was also one of the first questions we asked of them upon their return to Earth. According to my friend (who I'm not identifying out of respect for their privacy), the, uh, 'interpersonal relations' are in fact both manifest and juicy, and are a very substantial part of what they cover during the extensive surveys and debriefs that take place during and after the mission. It's a big part of what they're studying, even if they don't talk about it publicly.

They might send some porn and maybe give the crew some privacy.. but really, if they are focused on how not to die in the next 72 hours for the duration of the mission, this might not be as big a problem as you think.

I can't help but feel we're all underestimating boredom in this scenario. Yeah, you have to survive, but eventually things will be stable enough for there to be SOME downtime, with very few options for entertainment. When sanity is involved, boredom is a massive problem and sex might be one of the easiest forms of entertainment to arrange depending on what constraints were involved when planning the mission.

But then again, if you have a high probability of dying in the next 72 hours...

Here's a very good wrap-up of the issue: http://motherboard.vice.com/read/sex-in-space

At least in The Martian book/movie there is something about the topic.