Here is why:

Firsty, this is an unclear stance. If we'd interviewe women about feminism, I think the vast majority would say they are feminist. And yet, if we'd ask them precise questions about what they want to defend or fight for, there would be many different answers. Of course there are official definition of feminism. But I think there are as many differents feminist as women.

The same thing could be said about anti-feminism. On this subbredit I saw many posts wanting to criticize feminism or people saying they are against feminism. I did not discuss with these redditers. But I think that if I'd ask them to precise their opinions there would be many differents answers. And I'm sure many of these persons would defend some feminist ideas.

Wanting to fight against "feminism" is like fighting in a fog: it's not clear what we fight against.

Secondy, it creates a weak spot for a movement defending men.

I think we are all aware that promoting men's rights is a difficult task. When the quetsion of men's rights is evoked we are exposed to many fallacious arguments.

Saying we're against feminism is like giving a stick to be beaten with. This is a difficult enough fight, there is no need to give ammunition to opponents.

We can make mention problems such as the definition of rape wich excludes men rape or the empathy gap and deploy our arguments.

But saying we're against feminism and feminists is, in my opinion, useless and counterproductive.