Ignoring the question of whether it's abuse - which opens up the very silly door to an "I'm afraid of cats and all these nekos are abusing me" AR category - I try to think of it in the same manner as transsexuals. If you're a civilized person and if you encounter someone in RL who is clearly a man, but is dressed as a woman, and is introduced to you as "my friend Karen", then it's polite to refer to that person as "her".
Sure it might make you a little uncomfortable at first, but imagine the way she's felt all her life and how uncomfortable it must be for her every time she meets a new person and sees the "oh my God it's a man" look cross their face.
So if someone in SL wants to be a female avatar despite their XY status in RL, good for them. As to why? Who cares? Personally I think it's because most mens clothing sucks, and since we spend 75% of our SL time staring at our butts, why not stare at a butt you like.
If not knowing bugs you then don't sleep with them. In fact if not knowing bugs you you should probably not sleep with anyone since photos prove nothing, there's all sorts of software for altering one's voice to sound male or female over the computer, and if you demand a video show, anyone that isn't charging you US $1.95 a minute is going to tell you to stuff it.
About the only thing abusive in such situations is the number of times that it turns what could have been a pleasant frolic amongst the prims into a haranguing session to prove that the female avatar is being driven by a female pilot so to speak.