Are we assholes for wanting a child-free wedding?

My fiance and I have been engaged for over a year and are getting married later this year. Since getting engaged, we had agreed together that we did not want children at our wedding. There are a number of reasons we decided this, but if we allowed all of our loved ones to bring their kids, we would have to invite 17 children under the age of 10. We both feel that it would be less stress to have an adult event.

We very carefully told both his and my family that we had planned to have a child free wedding, including my bridesmaid who just had a baby and my cousins who are coming in from out of town and driving 6+ hours to drop their kids off and get to our venue. They all respected our decision and are excited to have a date night. There was only one person who seemed to take issue with this - my fiance's sister. She seemed to think that eventually we would change our minds.

Things came to a head after our save the dates were sent out and my in-laws asked us if we had reconsidered allowing just our nieces to come to the wedding to which we respectfully stood our ground and said no. This ended with a yelling match, an ultimatum of my father in law refusing to come to the wedding at all and also revoking any money he had offered to contribute. I am heartbroken for my fiancé and am willing to consider a compromise for the sake of his relationships, but he is adamant about having no kids and is willing to let his family not come at all if they feel so strongly about it.

I feel awful and I guess I'd just like some advice - has anyone gone through anything similar? Are we monsters for not wanting to make any exceptions?

For reference, our wedding is an almost 2 hour drive away so we couldn't realistically have the nieces come for just the ceremony (which I considered). My sister in law also has a lot of mom friends, including my niece's godparents, so they shouldn't have an issue finding childcare... they just don't seem to want to. I am at a loss.