Whilst my eyes did bulge in surprise at the op whatthenannysaw I can't help thinking that, if you were a guy, who'd employed a male gay nanny and post the same topic, I'd be angry and frothing about you being pretty much homophobic (in its truest sense, ie unfounded irrational fear of, rather than being a homist cock) so I guess I have to feel the same way about your op, even though you put it gently and are, after all, being honest about how you feel... which is valid, no matter how liberal you are in the wider world.
I think most people, whether gay or not, can find attraction in others, we are all sexual beings. For what its worth OP I would probably feel the same, the sexual being comment is important, and actually I don't think sexuality is every simple as gay or straight, or bi, there's a huge spectrum in between.
If it were a hetrosexual male you'd employed, being hetro you'd probably have felt the same if they were completely unattractive to you. It is your issue though, and ego IS involved a little, but you're not being unreasonable to wonder about these things, although it is your issue to resolve within yourself.
Worst case scenario, you hire someone you find attractive (whatever gender, whatever sexuality) there's nothing wrong with finding anyone attractive, there's nothing wrong at all in that, the world's damnwell full of them (and often they arent of the gender I usually expect) but if you have respect, firm lines in the sand, get rid of the irrational fear I think its workable.
Whoever you hire, there's always a risk, in your home to get lines muddled, whether its worrying about your DH fancying the young au pair, or your lesbian nanny fancying you, or the gay nanny fancying DH, and vice versa... there's nothing wrong with acknowlegding it. Calculated risk.
If you're committed to your marriage and DH, the lines are secure even if you find your nanny attractive (she probably can't be arsed) or she you.
I was a bit scared of lesbians in my fear and niavity when young, some of it was ego trip, some of it was ignorance... turns out 2 of my school friends are and always have been lesbians, and wouldn't touch me with a barge pole, certainly not without my consent.