How do I tell my boyfriend that he is being immature about something without him going stupid about it?

So I have been having a problem over something immature that my boyfriend has a problem with (a dumb problem he has with types of clothes - braless - which I don't think he should have a say in what clothes I wear) but everytime I try to talk to him about it, he acts even more immature and childish about it. It's like I can't even talk to him about the topic and it seems so dumb to me!

Why so immature about something so minor and stupid

How do I tell my boyfriend that he is being immature about something without him going stupid about it?
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