I think you are being far too hard on yourself.

It does go against social norms to ask about a man's penis first thing, but it doesn't mean you are a bad person. You're human and made a mistake because you honestly didn't know better. I've made many social goofs myself.

Part of it for me is that I was raised in a home with no filters where people say exactly what they think. So for me, that's normal. I only learned when I went to college that other people don't say things like that. It's the same with you and your curiosity about penises. You are just repeating what you learned at home. And that is natural.

I think a therapist might help you figure out what the social norms are and how to follow them. There are also support groups (some virtual) where you don't have to give your real name. I go to a lot of those and only share my first name.