Sam would never call Dean a girl because Dean is actually insecure about his masculinity, but it's okay for Dean to do it to Sam because Sam doesn't have that particular anxiety. This is contrary to popular fanon, but personally I've never gotten the sense that Dean thinks he's stupid, or that he seriously believes Sam thinks he's stupid, or that there's anything regarding this whole issue that keeps him up at night.

That's a really interesting point that I never thought of. I think you're right - Sam never teases Dean about his masculinity, and I agree with your reasoning of why.

But I think I disagree about Dean not thinking he's smart, though. For example, his comment last season about being a "grunt" and Sam responding that he's a genius (as well as Dean's facial expression when Sam said that). There have been other examples too over the years. I don't know. Sam's line in this ep bothered me, and though I like your explanation for it, I'm not sure I agree with it.