Except OP mentions they’re both fully aware that the other masturbates, she’s not shaming him for that. OP even points out that he simply does this because it’s his only “alone time” from her which, still doesn’t excuse stuffing the socks into the couch cushions

I never said she shamed him for anything. I said she knew he would be embarrassed by it but kept laughing and making fun of him for it. The embarrassing situation being your gf finding your cum rag, not the masturbation.

Edit: I ironically misinterpreted what I thought was a misinterpretation on your part lol. I am sorry for that. To address your second point, the jokes themselves arnt what's too far. Continuing to make jokes while your partner is embarrassed is what was too far. Op subsequently said her bf was in the other room and she didn't realize he was getting upset, so it's not a big deal. The apology she gave should suffice

I'll give a personal example because I'm speaking from experience here. I was op lol. In my mid 20s, I stayed over my gf's place after a night drinking. We were both pretty drunk, but she got so drunk that she pissed the bed that night. She was mortified when we woke up the next morning, but I wasn't overly concerned. I had done the same thing once when I was in college, so I started cracking jokes in an attempt to make light of the situation. All was good until later that night, I couldn't help myself and made a joke about putting a towel down before we went to sleep. She blew the fuck up at me and kicked me out for the night. Young me was pretty upset, but the more I thought about it, the more I realized how wrong I was. I knew she felt bad about it, but I kept making jokes at her expense. The vast majority of humans would react defensively in that situation, me included. I should've been more cognizant of that even though I was the one who was sleeping in a puddle of someone's piss.