I’m sorry you’re going through this.
It is the natural instinct of a mom to always love and support her child. Your son is always going to be your son regardless of whether he’s gay or not. So I totally understand where you are coming from. It should be your choice what you yourself do and if you need advice it should come from your priest, not from your MIL or from strangers on the Internet here.
At the same time, it’s understandable that a parent who strongly disapproves of his child’s lifestyle and future spouse might not want to go to the wedding or support it. You can’t make your husband go and if he doesn’t want to then you need to respect his choice just like he needs to respect yours.
I would add that many Catholics feel strongly that a Catholic shouldn’t go to a gay wedding or support it by sending a gift, etc. And many other Catholics feel strongly that if the wedding involves someone in your immediate family, then you can and should go. So there’s no one answer on this.
I hope you and your husband are able to respect each others’ feelings on this and accept it if you have a difference of opinion. I don’t see why one issue of disagreement should mean the end of a marriage unless there’s additional stuff going on. I would suggest counseling. God bless.