Of course it's a divergence from societal norms. That's just a statement of the obvious though, rather than providing any sort of reason. If it wasn't a divergence from societal norms, it wouldn't be an issue at all.
Tomboy only applies to younger women. However, a "tomboy" is certainly more accepted than an effeminate young man. Why is that? Both are divergences from societal norms, yet one will be significantly more ostracized than the other. Why is that?
Because its a "mans world" and men are expected to be "manly"? IMO, in our "mans world", being an effeminate male (even if straight), or being gay, is a sign of weakness when compared to the male norm, not because we think that person wants to be a female and we think less of females. I also think our male dominated society, on some level, finds female homosexuality to be arousing, thus more acceptable, but that may just be me.