Value and morality systems are put to the test under stressful conditions like longterm spaceflight. Your extrapolation of the potential problems that might arise when members of our puritannical society are put into the pressure cooker of a long flight is exactly right. Our expectations of how relationships work is partly premised on living in an open-ended world in which you can get away from the people with whom you can't get along, and go looking for others more congenial.

People have grappled with this. One answer was to choose only married couples for longerm flight, and pretend, as we do on Earth, that a wedding vow instantly eliminates all temptation and attraction to others. Then there's the simple logistical problem of finding bonded pairs in which both are fully qualified for the flight.

A mix of couples and singletons is very unstable. Skip it.

A group of singletons would have similar problems to the married couples, but without the added layer of betrayal-of-vows to contend with. But it's still prone to all the other problems.

Then there's the full-out "free love" crew. Humans have an easier time sticking to a vow to not form attachments, than they have sticking to a vow of monogamy. If the crew is generally psychologically bonded, with no odd-man-out individuals to form a nucleus of discontent, the crew might view sex as a social ritual, a comfort and a connection to humanity amid the vacuum. An ethic would develop that would gently discourage sexual jealousy as destructive to the mission.

As with any utopian scheme, of course human perversity will rear its ugly head. Attachments might form in defiance of the social norm; the human heart is unpredictable. That's why I stressed "generally psychologically bonded", so that the group can deal with the problem constructively, talk the jealous person down, perhaps use the rituals of sex to "seduce" the problem crewmember out of the jealousy.