While it is, of course, ridiculously hypocritical that anyone on the alt.right (or even those who decline to criticise them) should criticise Colbert for using an insult because they say it is politically incorrect, it's still true that Colbert should apologise to his audience for using an insult that no one should use.
There's nothing politically correct about my attitude to this: I've always disliked the insult because it is juvenile and can be readily be taken by any truly homophobic and violent gay basher to be sympathetic to their attitude.
Update: reading my link above, which seems to suggest that Colbert should never use any form of gay jibe for the relationship between Trump and Putin, I think that is a step too far. There are fine lines to be drawn here, to be sure, but I don't really have a problem with a comedian mocking Trump and Putin by pretending they are gay lovers, especially given their reputation as uber heterosexuals (and in Putin's case at least, outright homophobic.) But an insult based specifically on a homosexual sex act is different - it suggests (as Putin himself presumably thinks) that a particular sex act itself is degrading, but only when done by two men. By specifying it as being about a homosexual sex act, it is too easily read as either directly or indirectly mocking homosexuality itself.
I suspect Colbert will apologise.