I read the OP to be about taking underwear to wank into without the knowledge of his partner, who was upset enough to dump for it. So in that context, yes, I'd have a very big problem with it, just because I'm in a relationship with someone does not give him automatic consent to take my clean/dirty underwear to use as a wank into. The posters who have used the words creepy and pervert were clearly referring to the lack of consent and the example in the op. It's not a bit much to call men who don't seek consent, creepy perverts.
It's also ok to not be ok with it even if asked. If a man (or woman I guess) isn't comfortable with asking their partner if they can wank into their knickers because he is worried he will be kink shamed then he knows full well there's a strong chance of being told no and if he goes ahead and does it anyway then he's a creep. The wanking to possible vulnerable women in porn isn't more acceptable to me, I find it weird so many people seem ok with that too but I get called a prude for that opinion.
My old close friend had a partner years ago who she learnt was stealing her dirty underwear and sharing photos online with other men after he'd wanked into them. Not long after she moved in with him and they were splitting chores with him doing laundry, some of her underwear started going missing after going in the laundry and she joked with me that she thought a pervert neighbour was taking them off the line. Nope turned out it was her pervert partner who as well as posting photos online would steal her dirty underwear to swap with the dirty underwear of other men's partners. She found out when she was looking for her phone charger and found a jiffy envelope with some other woman's photo and dirty knickers in which led her to check his browser history and messages where she saw what he'd been doing.
Apparently it was why his two previous relationships ended too. He left that bit out when they met though. My friend was very open minded and would have been ok with him using her knickers, she's consented to his other kinks, it was that he didn't ask permission and he knew how important consent was to her due to her past, when asked why he never asked her if it was ok, he said the thrill came from knowing her and the women not knowing and that was a big red flag for her and she ditched him.
It's very different to someone consenting or willingly handing over their knickers to someone to wank in.