AITA for not stopping my son from making rude comments about his dad’s husband?

I (42F) share custody of my 16-year-old son, “Zach,” with my ex-husband, “Mark” (45M). We divorced when Zach was 11, mostly because we had grown apart and Mark came out as gay and cheated on me. It was a shock at the time, but I’ve made my peace with it. He’s now remarried to his husband, “Daniel” (39M), who is the person he cheated on me with.

The thing is, Zach doesn’t like Daniel. At all. I think he sees him as the reason our family fell apart. Over the past year, he’s started making sarcastic little comments about Daniel when he comes back from weekends at Mark’s.

Sometimes it’s kind of funny. Sometimes I laugh, or I don’t say anything. A few times I’ve just rolled my eyes and changed the subject. I didn’t think it was worth making a big deal over—it’s just venting, right?

But last weekend, Mark texted me upset. Apparently Zach made some joke to Daniel’s face and Mark lost it. The joke was “The only thing you’ve ever committed to is lying consistently”. He said it was cruel and homophobic, and that he’s tired of Zach “being allowed” to treat Daniel like he’s a joke. He asked me point blank if I’d been encouraging it, and I said no—but I did admit I hadn’t really shut it down, either.

Now Mark is furious. He says I’ve been passively feeding into the problem by letting Zach be disrespectful without consequences, and that if I don’t start taking it seriously, he might start reducing visits until Zach learns how to behave. That feels extreme to me.

Look—I’m not trying to be petty. I get that Daniel is his partner and he wants respect. But I’m also trying to pick my battles with a teenager who’s already moody and bitter about how things turned out. I’ve tried talking to Zach in the past about being civil, but it usually ends in a fight, or him shutting down.

Now I’m wondering if I really have been enabling something harmful, or if Mark is just being too sensitive about the whole thing.

AITA for not shutting Zach down harder when he makes fun of his dad’s husband?