An absolute fact of life
Just like death and taxes, Whenever males and females are together for a lenghty period in an environment that provides the opportunity, sexual activity is likely to occur. Not certain in any single instance but likely and abstinence will require extreme disipline by all parties.
This is not to say that a mixed crew could not be assembled with the disipline to abstain on a single mission but as missions become more common, so will sexual activity.
On Mars, In space, in the barracks, on the battlefield, it will always happen.
FrankC (based on a lifetime of research)