mine insists he jerks off into my pillow case. I believe him

Frankly, it kinda is. Not that it isn't a dick thing to do, especially without bothering to clean up after himself, but it is pretty common behavior.

He's not cheating on you. He's not using someone else's panties. You obviously turn him on. If you don't like it, ask him to change how he leaves them, whether it be washing them, throwing them away, or whatever...

At least he's not using another girl's underwear as a "vessel for his semen." Nor another girl.

He is a nasty fck to leave them on the floor for you to find.

definitely not normal, but if it's his kink then decide to deal with it i suppose, could be worse? tell him if he's gonna do it to have the decency to wash it, have some respect at least.(hear no evil, speak no evil, see no evil)

Would you rather him do that in another pussy? He should at least wash them, crunchy? EW

He should use underwear you've worn instead of clean underwear, and he should definitely put them in the laundry when he's done. Clearly you turn him on.

It's not at all unusual. Look, what we do sexually if you think about it in a non-aroused state, gets into "The Icky Zone" pretty quickly. Your panties are on his favorite areas of your body. BUT he damn well should launder them out of basic respect, tell him to quit being such a slob or he'd better invest in some hand lotion.

Seriously that is gross, but on the bright side there are 2 positives. The panties are in fact yours and also he isn't spilling his seed inside some other woman.