What arguments against couples in space ?

I just saw Mission to Mars, which is a good remake of 2001, just a few
more nits, and they use a face on Mars, not a monolith by Jupiter.

But nearly the same space suits and basically the same craft, a couple
more caracters, and some more acting, which 2001 could have used.

In the first couple minutes, where they have a party (on the last night
before the take off, that's one of the nits), someone 'can't beleave
they'll let couples on the flight'.

What real arguments have been fielded against that ?

Other than 'I don't like it', 'Its not appropriate' and similar ?

Regards

Carsten Nielsen
Denmark