Scarleteen

I’m very worried I have a weird fetish. I’m 15 years old and this spring my mom let me wear a
thong swimsuit for the first time. We live in Florida and she has seen so many girls my age wear them all over. Anyways, it all started when I rolled onto my tummy to tan my backside and my heart was racing fast because I felt so horny with the warm sun on my butt. I started squeezing my butt cheeks together really fast and a minute later I had an amazing orgasm in our backyard.
The problem now is I can’t stop. Almost everyday I now love to sunbathe on my tummy because it feels so good. My mom and one of my friends asked me what I was doing and I told them I was working my butt muscles and they didn’t seem to care. I’m worried something is wrong with me because the orgasms feel better this way than masturbating in my room. Do I need help?

Something that feels good like this isn't a fetish. It's just something that feels good. (A fetish, long story short, is an erotic focus on an inanimate object.)

It's fine to masturbate this way. However, what's not so fine is to masturbate in public, just in terms of having good boundaries where other people are concerned. Masturbation is something we need to keep private, or only for sexual partners who have consented to be present or part of our masturbation. So, if you're in your backyard all by yourself, that's cool. But if and when other people show up, you need to respect their boundaries and save masturbation for when you have privacy or consent.

Now I’m nervous because I have definitely done it at the beach. Usually I make sure no one is near me and I discretely squeeze my butt muscles and orgasm pretty fast. I also did it with my best friend right next to me but she didn’t notice. Sometimes I get horny at the beach and I can’t help myself. Before I started wearing thong swimsuits I never got horny at the beach. What should I do?

You should respect other people's boundaries and right to consent to be part of sex or your masturbation and save it for when you're alone or unless people consent to be part of it. That's just common courtesy: it's not okay to make someone part of your sexual experience or scene without their express permission.

Just because we feel a desire to do something sexually doesn't compel us to do it: you can control yourself in this way, we all can. And if you don't feel like you can control yourself in that way, that's something you'll need to learn to do, Dave.