What is offensive is a fuzzy line sometimes. When someone is close to a gay man and teases about a gay ass hat, it is probably loving and witty. If a homophobic man says that in disgust, a slam. It is all about intent for some things.
Transvestites are usually gay men who dress as women and not always transgender. Straight men can be cross dressers and usually in private but not always.
If any term his hurled in anger or disgust, even if the word is not a slur in itself, like transvestite, it is wrong.
Be safe and say nothing offensive ever. I have close gay friends and with them I have the familiarity to say something like gay ass hat and they would find it a warm and witty remark.
Good advice...it's what I try to do but am naturally curious as what is ok and what's not. I've been just as confused when I hear black people use the N-word. I think I'll put it in the same catagory as politics and religion: avoid it as much as possible.