I think that it is important for the PA to set boundaries. I was a personal assistant once, male, to a female. We never had any problems, but that was probably because she was 10 years older than me or something. It could also be because I took some classes on being a personal assistant and because of this site
on how to be a good personal assistant.
Was the question hypothetical, or are you trying to decide whether or not you will become one?