I know that this is hard, but try not to take this personally. The worst part of this is that he stole your mom's panties. The panty fetish, in and of itself, is not wrong. Panties (well, some at least) are really sexy; gorgeous, lacy, feminine, and worn near the most intimate part of a woman! Even the word "panties" sounds sexually exciting. So I think your bf's love of panties stems from his attraction to women, of which you are one.