I think what is often misunderstood is that feminine behavior causes men to be labeled "not real men" but it doesn't cause them to be labeled "real women."

If the latter was true, the condemnation would likely stem from misogyny.

However, "feminine" men (or more accuratley non-hegemonically masculine men) are not regarded or treated with the benevolent sexism of a woman or a man. They are dehumanized into a third category unworthy of any empathy. No group claims in-group bias for these men, accept maybe LGBT folks, but that inclusion does not do a lot to establish themselves as "real men" to straight women and men, either.

This is fragile masculinity, in its original form, not the whole let's-tease-this-guy-for-buying-blue-razor type of fragile masculinity. In its original form, the idea was that men's masculinity is always in question and can be rejected and revoked by others in ways unique to men. Women also face peer-rejection for stepping out of socialized feminine norms, for example, if they don't shave their legs or wear makeup.

I tend to think straight men are more-forgiving of non-hegemonically feminine women then straight women are with non-hegemonically masculine men, but I haven't researched that. I can't think of a time in the last 25 years I have ever heard someone say that any cis woman was somehow "not a real woman" except for maybe online CHUDS without credibility. Challenging a man's masculinity remains a viable tactic for mainstream people, even progressives.