i think it could get very complicated.
on principle, no i have no objections, as long as she was qualified to do whatever her other duties were, cleaning childcare etc, i would not allow an aupair to have male guests stay overnight. (my house, my rules.) with a lesbian, i wouldnt want her to have her girlfriends staying overnight, but, how do you tell a girlfriend from a friend who is a girl? hence the complications.
however, this is all theoretical as i would never have an aupair for a gazillion other reasons.