But Nasa officials are starting to debate the unofficial in-flight celibacy ruling in preparation for an unprecedented three-year journey to Mars, which the space agency is planning for the next century.
Thus far, men and woman, even husbands and wives, have flown together in space, but there have been no known cases of heavenly bodies giving in to earthly desires.
In 1982, Svetlana Savitskaya slept in the same apartment as the (male) crew of Russia's Salyut space station, but this was a platonic affair. An American married couple flew on the same mission for eight days in 1992, though the chances of any conjugal relations developing on that short trip were diminished by the fact that while he was working she was sleeping and vice versa.
The sheer length of the Mars trip, provisionally booked for 2012, is prompting some astronauts to question whether they can remain celibate that long. Michael Foale, the British-born American astronaut, says he wants to take part in the Mars journey, but not alone: "I would like to go with my wife."
Dr Al Holland - a leading Nasa psychologist - concurs, saying that for these long hauls, the space agency is going to prefer "stable relationships" between the crew rather than chilly professionalism.
Celibacy does have its backers, however. Russian cosmonaut Musa Manarov, who spent more than a year on Mir during his first mission in 1987-88, says: "A young guy could hold out three years without women." Laura Supra, 29, youngest member of Nasa's longest-duration test on Earth, says: "That's one of the things you give up by going up there."
Doug Cooke, head of Nasa's space exploration office, warns that if astronauts get pregnant, they will not have an option of turning back.
The journey will take 180 days, followed by 500 on Mars and a further 180 days for the return voyage to Earth.Reuse content