I used my sons girlfriends underwear
Why would you use your son's girlfriend's underwear?
This could be the only time I get to see her underwear up close.
It is wrong to steal personal belongings but it is especially creepy with it being your son's girlfriend. It sounds creepy and terrible and you shouldn't do that.
It's just a pair of underwear.
But the kind of message you give off for stealing a pair of underwear is not a very good one. This makes you seem almost like a pedophile in a way since it is creepy to do something like this.