Free-floating sex could be physically dangerous, with bodies ricocheting off walls, striking body parts; if "decoupling," partners could go shooting away from one another and colliding with equipment.
Sex in space also entails the risk of penile fracture. Excessive lateral or downward buckling might result in tear in the fibrous outer tissue of the penis. Symptoms include a sharp snapping, cracking or popping sound, excruciating pain, swelling, bleeding, and deformity of the penis. Treatment would be cold compression pressure dressings, splinting, analgesics, and surgery to correct the tear, but it's unlikely because of the lack of proper equipment and surgical expertise.
How might sex in space work? Restraining one or both partners by the use of footholds, belts or cords, with the other partner loosely bound; or might involve a 3rd person as helper. (I will now pause for consideration of the kinkier aspects of this. ..... Ready to go on?)
It's important to contain the fluids generated during sexual activity, such as perspiration, saliva, hair, semen, and vaginal mucus. In microgravity, these form globules that float and can be inhaled. Body heat dissipation can be a problem, especially during close contact. Noise and unpleasant smells inhibit sexual desire, as well. Overall, sex in space is possible with proper precautions.
Psychological effects are also potentially serious: jealousy, love triangles, interpersonal conflicts exacerbated by the isolation of space (incidents in comparable situations, Antarctica). Consider the mission consequences of a "soap opera in space."