Whenever I've lived in I've had house rules, and adhered to them. Especially regarding no unknown overnight guests, no endangering the household whilst shitfaced etc. It might be "my home" as I live in, but it's my employers property and their rules have to be respected and obeyed (as long as they aren't too ridiculous, ie in bedroom by 9pm on a weekend).
As far as sexuality goes... I think personally it's none of anybody else's business who floats my boat. As long as I do my job well, and respect my employers, I demand the same respect and trust from them in return that I am mature enough to live my own life as I see fit. If they feel I am "good enough" to help in the raising of their family, then they will accept my life choices/preferences. If, however, they base their offer of employment on certain criteria BEFORE offering the job, then that is their own prerogative. So if they don't want a gay childcare provider and state that in their requirements, I believe that's up to them. They may well get flamed for it. But I would rather know the values/opinions of future employers before the fact as opposed to after it. Just as if the employers themselves were gay, I would rather know beforehand but it would never occur to me to base a decision on that fact alone. I would happily work for a gay couple/single parent as I think it swings both ways: mutual respect for one's own life choices.
I know sexuality has no bearing on someone's ability to do any job, but the truth is people will always have opinions and preferences. If someone would prefer not to have a gay au pair, then let them get on with it. There's no point in getting all het up about it, that's not productive and in the grand scheme of things it's not going to change anyone's mind just because a forum gets up in arms about it. As long as people express their preferences with tact and sensitivity, then who are we to judge? Live and let live.