If Shermer really did rape anyone (by the conventional legal definition of rape) then I make no excuses for him.
In fact, even if he only did what he's specifically been accused of so far with at least partial corroboration (that is, on multiple occasions getting women drunk by buying them drinks and then cajoling them into his bed), I think that would make him a jerk, and more than a little bit pathetic, and I'd be disinclined to ever have any social engagement with him. As James Stewart said in The Philadelphia Story, "There are rules about that."
On the other hand... rules, not laws. From the evidence available so far, what Myers and his anonymous correspondent appear to have done is apply their radicalized definition of rape (which includes consensual sex when there was any significant degree of voluntary alcohol intoxication involved) to the real world; specifically, to the public accusation of a specific person.
What, there can be consequences for that?
He's violated one of the cardinal rules of slacktivism: once you've redefined everything to your liking (such as what rape is), don't go and break the spell by actually putting those new definitions to the test in the real world. That only works if you can succeed at getting laws changed, which in turn (in a representative democracy) requires debating the matter with people who disagree with you. Yuck! Who wants to stoop to that?
Their PR defense against the accusation of defamation appears to be to pretend that any questioning of whether the legal definition of rape actually applies to the extremely sketchy scenario revealed so far is actually dismissing the word of one or perhaps several women that anything untoward happened at all. Hence the "how many women's word does it take over the word of one man" rhetoric, as though that, rather than whether there was anything close to sufficient reason to justify the words "I was raped" by a specific individual in a public accusation, were the main issue.
I don't think, though, that "to us, 'rape' has a different definition" will prove much of a defense. Lawyers have a stubborn tendency to focus on the real world.