AITAH for not letting my boyfriend’s children use my snorkel set?

Background - my boyfriend (M, 50) bought me (F, 45) an expensive goggle and snorkel set for the Christmas that’s just passed. It was my main Christmas present from my partner (we’ve been together six years and both have teenaged children from our previous marriages). I’ve told my partner that I do not want anyone else to use my snorkel set - a) because it’s gross to share things that you literally put in your mouth and suck on, and b) it’s special to me, because it was a gift from my partner and I don’t want it broken or really just used by anyone else. Today at the beach my partner told his daughter (11) that she could use my snorkel set. She has her own here at the beach, it’s just not quite as good as mine. I said no, she couldn’t use it. He said I’m being an arsehole, and I said he was being an arsehole because I was very clear up front about not wanting to share this one thing (I don’t have any issues sharing anything else, or nothing that’s arisen so far). I have a very good relationship with his daughter and she didn’t care about not using my snorkel set. So - AITAH?