"I would assume that what astronauts wear when they sleep would be adequate to prevent uncontrolled release of semen (thinking only of men, but women may also produce fluid). So the idea that astronauts who "experience nocturnal emissions" would not be chosen is unlikely. These are not normally frequent.
Most men do not experience nocturnal emissions except from relatively long abstinence from sexual activity (including masturbation). So they wouldn't know until they are abstinent for a substantial time.
I found a NASA paper that mentions nocturnal emissions:
http://ntrs.nasa.gov/archive/nas... (page 37)
Men are apparently able to find successful means to release tension under conditions of long-duration confinement. Sexual tensions are an important practical example. Cramer and Flinn (ref. 66) found that, while all the men had sexual fantasies, it took 10 to 14 days before these became frequent. Before this time, the men were able to sublimate their erotic feelings into the mission. The sexual fantasies apparently developed as sublimation became less effective. In addition, three men experienced nocturnal emissions.
66. Cramer, E. H. ; and Flinn, D. E. : Psychiatric Aspects of the SAM Two-Man Space Cabin Simulator. SAM TDR 63-27, Sept. 1963
The sample, then, was 8 men in 2-man simulations, so this would be 16 men. The duration was not described.
If it is considered desirable to avoid nocturnal emissions, which are relatively uncontrolled and predictable, periodic emission by penile stimulation could be used. Contrary to much that has been said, this could easily be managed with no risk of release of floating semen."
https://www.quora.com/Do-astronauts-experience-wet-dreams-during-their-stay-in-space 
