Not as easy as it might seem, but NASA is gearing itself up for tackling it:
We cannot expect astronauts to spend three years in a spacecraft and not have sex - of some kind. Probably with each other, and likely in more than one combination. Sex in space presents a number of challenges beyond tangible matters like zero gravity or awkward enclosures. In fact, the physics should be the least of NASA's concerns.
It's the touchy-feely bit that the agency will need to consider seriously. Blindly applying Earth-bound standards that astronauts cannot follow under space-voyage conditions will only lead to guilt and shame. What happens if one person in the spaceship makes a romantic overture, only to be rebuffed? What happens if an astronaut in a relationship back home falls in love with another member of the mission? What to do -- send a breakup text message from Mars?
How do you handle love, sex, romance, heartbreak, jealousy, hurt, unrequited longing, crushes, loneliness and twitterpation when you're 18 months away from Earth and perhaps unsure whether you'll make it back?
And what happens to the lube?