I found this journal article via Instapundit: Sex On Mars: Pregnancy, Fetal Development, and Sex In Outer Space (http://journalofcosmology.com/Mars144.html)
It analyzes what would happen on a three-year round-trip voyage to Mars if both men and women are included in the crew.
The author uses analogies drawn from primate behavior and from studies of human behavior during isolation experiments and teams of researchers in the Antarctic. The conclusions drawn are basically that if you put a mixed-gender group together for long periods of time, they're going to begin pairing off and having sex, no matter their marital status, the agency's rules, or their training. There was even one pretty depressing anecdote in which two male astronauts began fighting and one of them then attempted to rape a female astronaut during a 110-day isolation training exercise.
Here's the part that I found most interesting:
When a woman chooses to have a sexual relationship during her sojourn in the Antarctic, it is often with senior (rather than junior) personnel, including the station leader who is usually seen as having the most status by the women, as well as an unfair advantage by the other men, thereby creating considerable tension and conflict (Stuster 1996).
On the other hand, most studies report that including women as team members has a very positive effect on morale (Rosnet et al., 2004; Leon 2005), especially if they are married and accompanied by their spouse (Leon 2005; Leon et al., 2003). For example, in a study of three married couples from different countries icelocked on a boat in the High Arctic for a 9 month period, partners provided each other with significant emotional support. Married partners were also instrumental in alleviating interpersonal and group tensions, and contributed to the effective functioning of the team (Leon et al., 2003).
Naturally this all got me thinking about Trek. It's no wonder Archer says in "Broken Bow" that Starfleet has no official rule against dating! By his time they would know from experience that such rules won't work.