Libido can affect behaviour, judgement, group dynamics etc, which could pose a risk for the effectiveness of a mission. Is this somehow addressed by NASA, for example in screening during recruitment, in training, in space-lifestyle design or in medication?
Sexual psychology in space could in fact be an issue, as you have surmised.
In Space Psychology and Psychiatry by Nick Kanas and Dietrich Manzey, there is a section on gender issues which states:
Rosnet and colleagues  found the presence of seduction behavior, rivalry, and sexual harassment in their polar station when the women were about the same age as the men.
The possibility of pairing and sexual contact also needs to be considered during long-duration space missions. Will such activities lead to jealousies and problems in crew cohesion? In a recent space simulation project conducted in Moscow that involved several multinational teams of isolated and confined individuals (called SFINCSS, or Simulation of a Flight of International Crew on Space Station), a female participant reported unwanted sexual advances (including kissing) from a male participant. This resulted in a breakdown of cohesion and group rancor that affected not only the isolated teams but also the participating agencies [Inoue et al., 2004; Kass and Kass, 2001; Sandal, 2004]. Stuster  has pointed out that similar unwanted sexual attention has occurred during Antarctic missions, and that disruptions in cohesion have taken place as a result of male-female pairings. He also stated that if a woman chooses to have a relationship during her stay in the Antarctic, it often is with one man, with a preference for senior over junior personnel. Although the other men usually accept the situation, disruptions may occur if the relationship involves the station leader, who is seen as having an unfair advantage. Along these lines, it is interesting that in the days of the polar explorers, the commanding officer of the ship or the expedition leader was permitted the luxury of taking his wife or mistress with him on the long voyage [Stuster, 1996]. Buckey  has reviewed a number of sexual and non-sexual tensions that might occur in a mixed-gender crew going to Mars. He suggests that the crew-members could be observed under isolated and confined conditions during training to see how they come together as a team in reference to possible problems with harassment, flirtatiousness, or jealousy. Should such problems occur, further training or even replacement of offending crewmembers might be necessary for the actual mission.
One might argue that future crews should consist of married couples or stable male-female pairs in order to minimize competition and conflict. However, there is no reason to expect that such a crew composition would prevent secret liaisons and jealousies, since infidelity and extra-marital relations occur on Earth in less stressful interpersonal environments. Enforced platonic relationships and sexual abstinence also is a possibility, but it is difficult to imagine this as a realistic scenario for healthy energetic people who are confined together for long periods of time. Perhaps novel social systems and customs will evolve in space that are similar to those found in communes, where pairing and unpairings will be tolerated with a minimum of conflict and animosity.
From this it would seem that there is currently no existing policy (beyond whatever sexual harassment policy we can assume to be in place), but that this issue is one under heavy consideration, especially as the time of human interplanetary travel approaches.
What we can be sure of is that this issue is under study, and that the astronauts are given counseling on top of their extensive training. The screening and training process probably minimizes the issues that are experienced, but it has been noted that even in these conditions suppressing such issues only postpones an inevitable reaction of some form. Instead, it would seem that healthy outlets need to be provided.