Saying the words penis and vagina

So I'm only a teenager, but my brother is 12. The other day I was arguing with him about a pair of shoes (he basically said they were boy's so I couldn't wear them cuz I'm a girl) and I said "what's it matter, the shoes don't have a penis do they?!" And he got like embarrassed I guess idk he was so mad though he mumbled under his breath but it was only because I said the word penis. Like I tested out my theory later and did something similar again when we weren't arguing we were just talking about laying on our stomachs, and I said "well when you have boobs, sometimes it hurts" and he again completly shut down. Because I said the word boobs. I bet he'd fait if I said vagina, so I'll spare myself the trouble, but why is this? What's wrong with those words? I feel much better saying penis than I do saying ding-a-ling!