In a string of insults against President Donald Trump last night, comedian Stephen Colbert lined up what he surely thought was a great, hilarious diss. Instead, he blurted out some homophobia.

Colbert was in the middle of a monologue launching various insults at Trump, including some fat shaming, “presidunce,” and “pricktator.” In the course of this, he said, “The only thing your mouth is good for is being Vladimir Putin’s cock holster.”

This is a kind of insult that has been used against Trump again and again. From murals of Trump and Putin making out to late night show gags, it’s now pretty popular among progressives to paint the US and Russian presidents as being gay for each other.

But the only way this works as a joke is by demeaning gay people. The underlying implication here is that gay relationships are somehow extra funny — that Trump engaging in sexual acts with Putin is hilarious because it’s gay.

In a setting in which Colbert is deliberately trying to find a way to insult Trump, it’s telling that he resorts to suggesting that Trump is engaging in sexual acts with another man. The suggestion is that the worst thing that could happen for these men is if they engaged in homosexual acts together, as if that devalues them as men, makes them submissive, or emasculates them.

This came up back in 2013 when Alec Baldwin — now also a progressive darling due, in part, to his anti-Trump comedy — after he called a photographer a “toxic little queen” and “cocksucking [inaudible].” Baldwin said at the time that he didn’t understand why his comments were homophobic.

CNN host Anderson Cooper gave a good explanation on Howard Stern’s radio show: Baldwin was in a situation in which he was trying to find the most insulting thing he could call a photographer he didn’t like, and he landed on characterizing the photographer as gay. “The worst thing you can possibly think of to say, which is what this situation was, to talk about a sexual act between two guys as being the worst thing you can possibly think of,” Cooper said.

Cooper clarified back then that he didn’t know if Baldwin is homophobic, because “I have no idea what’s in his head.”

Similarly, we don’t know what’s in Colbert’s head, and his comment was meant as a joke. But it’s not a good look for the comedian who’s fashioning himself as the anti-Trump progressive on late night.

Watch: A timeline of the 3 Trump-Russia scandals