My (23F) boyfriend (24M) is hell-bent on telling children the truth about Santa Claus

You might get to the end of this and think it's a big deal over nothing, but it's really upsetting me and I just want to get it out.

I started dating my boyfriend at college. Obviously in that environment, there aren't many children around, nor are people likely to spend Christmas away from their families. That's what we did for the last 3 years - our families live at opposite ends of the country, so we never mixed for the holiday week. This year we both graduated, and moved in together in our college city. Due to the world being how it is, we decided not to see either one of our families for Christmas, and are just spending it with each other.

The issues started the other week when we were out shopping. There was a socially distanced Santa's grotto in the large store we were in, and when my boyfriend saw it, he exclaimed in a loud voice "Santa isn't real!", and looked at me like I should be laughing or praising him. I didn't - I was shocked, and dragged him away before anyone could get to him (there were several angry parents waiting who were glaring at us). When we were outside, I asked him what the hell he was playing at, and he said that he's always hated the idea of Santa, and kids being "lied to" and "manipulated" into believing that some magical man bought all the presents for everyone in the world. I immediately asked if he'd had some traumatic experience with a Santa (like seeing someone part way through putting on the beard and hat) but he said no, and that his parents always told him the truth (he's an only child), and no one in his family ever tried to play along with the Santa "lie". This made me really sad - I loved Santa as a kid, and was lucky enough that I learned the truth about him/Tooth Fairy/Easter Bunny organically when I was old enough, so couldn't quite believe that someone had grown up without that. I was also appalled at the shouting, and told him that it was childish and stupid, and he agreed not to do it again.

I thought that would be the end of it, but no - the other day I was going to the mail box, when someone who lives in our apartment block came up to me. She said that she wasn't sure if I knew, but that my boyfriend had basically ruined their Christmas. She explained that the day before, he'd been out for a run and had passed her and her son (aged about 5 or 6) on the way out - they'd chatted for a minute, but it was obvious that the son wanted to leave and go back inside because he was "making cookies for Santa"... Yep, you guessed it, my boyfriend opened his mouth and told the kid the whole truth about the situation, right down to how if the kid looked around the apartment enough, he'd probably find that all the presents "Santa" was going to bring him were already there. That's exactly what happened - the mother couldn't really discipline the child because it wasn't his fault, and now a little kid has lost years of wonder because of my stupid boyfriend.

I confronted him when I got back, and all he could say was that he didn't think I'd be mad because he "didn't shout". I told him straight up that any future children we might possibly have had a right to be children, which includes being allowed to believe in Santa. He said that it would never happen. We're now not talking. I can't possibly comprehend breaking up over this, but it seems like an immovable impasse. Am I being too stubborn - is Santa really that important?

TL;DR - Boyfriend wasn't brought up to believe in Santa Claus and thinks it's "manipulative" to let children believe in him; will happily tell children the truth, and leave parents to deal with the fallout. Is this really what is going to end my relationship?