My (26F) boyfriend (31m) is being super insecure about me using women’s locker rooms, is this normal?

I am a swimmer, always have been. Recently my serious boyfriend (we live together) found out the showers at the pool I swim at are old school and are open/gang showers. He was horrified, saying my body should only be seen by him, and shocked I was comfortable with that. I explained I was raised with 2 sisters, was on swim team in high school and college, and that it’s all old hat for me. I dress modestly in public, and would never change around men, but I just don’t care about a locker room with other women. He keeps arguing I must be getting some kind of “kick” out of it, and also points out some women might be lesbians (well, yeah??), and even if they aren’t, that they are still “seeing me” when that should only be him. He’s REALLY intense about it, basically threatening to break up with me if I don’t stop showering at the pool/gym. I obviously love my bf more than swimming, but i’ve found his opinion on this so weird and offputting that it’s making me re-think him. We’ve been together almost three years. Thoughts?

EDIT: I am fully nude for the shower, but wear a towel around me to/from my locker, and change quickly and fairly modestly at my locker. I’m not the old lady eating a sandwhich in the buff.

EDIT 2: OK, I’m convinced, I’m dumping him. I’ve never broken up with anyone before (only been broken up with), and this is going to be tough because we live together, our families get along, etc, but I really think y’all are right about this.