About a year ago when one of my sister's friends was spending the night and they were at the store, I sneaked into my sister's room and pulled her friend's panties out of her bag.

I started to masturbate with them, rubbing them on my penis and smelling them, and I really liked it. I've done it quite a few times since then and loved every minute of it. I've even done it with one of my friends' mom's panties. I always put them back, and I never get any semen on them. Is there anything wrong with this fetish, or is it just kind of creepy? Should I stop?

There's nothing particularly wrong with having an attraction to women's panties – but the way you're acting on it is definitely wrong. You're invading these people's personal things as well as their privacy. Would you like it if you learned someone you didn't know was stealing your own underwear so they could masturbate with it? Saying "What they don't know won't hurt them" isn't enough – even if they never found out (and they certainly might one of these days), you are still messing with their private belongings, violating their personal space, and that's just wrong.

It would be harmless if you obtained a pair of women's panties (not by stealing!) and masturbated with them, using your imagination to pretend they're someone else's. If this doesn't do it for you and you can get pleasure only by using someone else's stuff, that suggests a serious problem. In this case you should seek out professional counseling, because it will almost certainly get you into major trouble sometime down the road.