The answer is..it depends. Depnds on how far the representation goes and what actions are being taken based on it. I know some ava's who say in their profile or otherwise tell--always ok. But I know some others who have also represented themselves as the opposite gender in RL as well, even using a voice pitcher to attempt to complete the illusion and using someone else's photos. Not cool.
Bottom line, we all have a basic right to chose who are sexual or otherwise intimate with. For some people, the gender of the other ava is a deal maker/breaker. Right or wrong, there it is. Some ppl can be hurt or even traumatize by a deep deception. I've seen it happen--more than once.
Many ppl want to simply point to the TOS and give a quick and decisive "nope" as an answer. No so fast please. Avatar's rely on humans to exist--not considering bots. Those humans have real feelings so, like it or not, there are some ethical obligations here (and perhaps some legal ones).
My two cents, I don't care about your kink--as long as you obtain consent honestly from one legally capable of giving it. If you have that, have fun!
For those of you for whom RL gender is a deal breaker, remember one of the cardinal rules of SL:
NO VOICE, NO CAM, NO CHANCE.