I would prefer to have an au pair who had a steady boyfriend/girlfriend, rather than one who had lots of casual encounters, for similar reasons to scienceteacher.
Earlier on this thread, someone said you could swap the word 'lesbian' for 'black' or 'jew' to see how offensive it was.
I don't think it's as clear cut. While your sexual preferences, like your race, are part of your identity that is not the whole picture IMO
Being 'black' is what you are, not what you do or what you prefer. Being a lesbian IMO is both what you are and what you do/prefer - by definition a lesbian is attracted to women more than men (AFAIK) whether this is translated into sexual behaviour or not.