Why is a man in his underwear funny while a woman in her underwear vulgar and ualised?

You greatly overgeneralize a very complex subject, a hot y man showing off his body is just as to someone who appreciates men as is a hot y woman showing off hers to someone who appreciates women, done differently it can be vulgar and offensive, even obscene, it's a matter of context and presentation. The same is true of humor, it's a matter of timing, context, and presentation, not simply of the gender of the people involved.