I think one reason men complain is because they hear mostly the loudest group (or at least this group stands out because of their views) - the hardcore, extreme, so-called "man-haters".
You know, the ones who say crap like "Wouldn't it be nice if there were only women in the world" or "Now that we have genetic engineering, we can finally be free of those animals!” This small minority tends to draw attention to itself.
If you feel like you are under attack, your basic response is to defend yourself or fight back.
Another reason is because of the perception of a double standard. The perception that feminist want equality only when it suits them. That they want all the benefits of but none of the drawbacks of being equal. I'm not saying that’s true, but that is an impression that many men have, and I have meet and known women like that.
I for one believe in equality, but I also think you can’t have it both ways. We’re either equal or we're not.
Also, I think some of it is just plain ignorance. A lot of men really don’t know what feminism is (or at least not the details) and that doesn't help either.
I myself have only a basic idea about the philosophy, but I've listened enough that I know that there are many feminist who are reasonable, logical people who just want a even shake in the world.