It's completely up to you to wear one. If your boyfriend really cares, he will take your perspective into mind and not argue with you to wear one. I can see how wearing a bikini can make certain people feel insecure about their body, but really, no one on the beach cares if you're fat or skinny since they're all having fun with their own friends. If they do that's pretty sad and immature. In fact, I would say more people would judge you for wearing shorts and a vest instead of a bikini. I see from his perspective how it doesn't feel very 'girlfriend-ish' to not wear something that is particularly suited for going to the beach, etc but in the end it is your choice. Hope this helps.