My stepbrother's assertive touch makes my cock hard and I struggle to resist orgasm

I've always been a bit of a rebel, and my stepbrother is no exception. He's always been a bit too assertive with his touch, especially when it comes to my girlfriend's body. I can't help but get a little hard whenever he's around, especially when he's groping my girlfriend's boobs. It's like a forbidden fantasy that I can't resist. I've been struggling to keep my cool, but it's getting harder and harder every day. Sometimes, I even find myself jerking off just thinking about it. It's like a game we're playing, a dangerous one, but oh so thrilling. I know I shouldn't, but I can't help myself. It's like I'm addicted to the thrill of it all. I don't know how much longer I can resist, but one thing's for sure, I'm sure it's going to end in a messy cumshot.