Firstly. Unless your bf is some bronzed toned Adonis then you will know as well as anyone that people stay in relationships because they find the
whole person their partner is attractive, not just how they look in a bikini. There are men who, objectively, you might find much better looking than your bf, aren’t there? But you wouldn’t just up and leave him for them, would you? Because gorgeous as those other men might be to look at, they aren’t your bf who you like for far more reasons, right? So, can you tell yourself that it’s exactly the same for him, however many women there are out there who are objectively better looking in a bikini?
Secondly. Maybe he does find her attractive. Well, you can worry about it and let it make you feel insecure and bad about yourself; or, you can say to yourself, so what? I’m also very attractive, and he’s with me not her, and he’s lucky to have me. In the second instance you feel good about yourself. In the first instance you feel bad. But whether you feel bad or good, the end result is still the same - he finds her attractive. So you may as well take the road to feeling good, because your feelings don’t change the outcome, do they, and if the outcome is the same then why not feel good rather than bad?
I recognise that that’s a fairly clunky train of thought to follow, but you can’t change anyone but yourself and when it comes to insecurity you just have to stop with comparisons to other people because it’s a negative circle in which you’ll always find yourself coming up short. Other people might have X. But you have Y and that’s just as good.