I have had an issue in the past where my avatar was dating a male avatar only to find out later on it was a RL female playing the character. Sad part of it was, the player of the male avatar had misrepresented themselves, even so far as to send pictures of their RL husband, claiming it was them in RL. I was led to believe in was a RL male playing a SL male and fell in love....only to find out later on it was a woman in RL and it hurt...ALOT. Even after finding out the truth, I tried to stay friends with the person, but there are just some things you can't forgive when your heart has been hurt.
Is is abuse to play the oppsitie gender of what you are in RL in SL? No. Is it fair to misrepresent yourself and hurt someone's feelings? Hell no.