Lawrence Palinkas, a medical anthropologist at the University of Southern California, has released a study of the sexual behavior in the closed confines of Antarctic Research stations. Due to the limited team size, close proximity and isolation for long periods of time NASA has taken an interest in the study.
With plans to return to the moon, and an eventual three year trip to Mars, the possibility of romantic relationships developing, and breaking apart, are a real possibility. Studies from Antarctica show that a 50-50 split between men and women in the crew is optimum. While sex can relieve boredom (and I wonder how much of my tax dollars were spent figuring that fact out?), there is of course the possibility of jealousy developing between team members. The biggest worry is an unplanned pregnancy, since no means to deliver and care for a child would exist on a long flight.
Also, to quote the article Sex in space: The new frontier in the California Aggie, "Then there are also technical problems with having sex in space, such as weightlessness and close physical monitoring by a station on earth, leaving astronauts with little privacy or alone time, she said."
Erm... I'll leave pondering the "technical problems" to you.