We disagree that this actually changes the meaning of Feminism.
Yes, some horrid people need an excuse to dish out abuse and Feminism often becomes that excuse (I'm thinking about everyone who's ever used the potential-rapist argument).
Does this taint Feminists, the group? Maybe a little, yes.
Does this taint Feminism, the idea that genders should be as equal as possible? Well, IMHO, no.
Feminism was about seeking equal rights and equal opportunities for women. In the West it largely achieved that goal (we're not there yet, but consider where we started). Thanks to tumblr, twitter and the blogosphere echo-chamber, the feminism most people become aware of online is very different from that.
Worse yet, there is no such thing as "the feminist movement" (anymore) by a long shot. It's become a broad term that encompasses everything from egalitarians to misandrists. And any criticism of the later group is always portrayed as a criticism of the former (which is why we end up with this entirely pointless MRA/SJW shitstorm-on-demand situation we have now).
Nobody is forcing egalitarians to call themselves feminists. But clinging to that label and pretending it hasn't changed its meaning is no different from trying to distance your communism from Stalin or your nationalist socialism from the Nazi party.
In the real world, it can be used as a fig leaf to justify a variety of abusive behaviors. For some people, that encompasses the whole of their experiences.