Hello Dragonfly. I don't think that you are being unreasonable to have that kind of feelings. You must be pretty upset over everything and I can understand why you would be. I sympathize with you on this very difficult situation.
I've seen my friends and relatives go through the same problems. Some of them also never want to see their ex's new partner. However if the new partner is a good person and the relationship becomes long term then the new person will want to be involved in what their new step sons or step daughters are doing. Whether that be recreational activities or activities like graduation.
I had one friend who never came to terms with his ex's boyfriend. When it came time for his daughter to graduate from grade 8, the daughter wanted her step Dad to be there as he had helped to raise her for 6 yrs by then. The young girl also wanted her dad there as well, but my friend refused to go. He also missed a lot of other events in the girls life. The person who was most hurt by this was the child.
As time goes on, if your ex daughter-in-laws new relationship becomes solid and long lasting, the boyfriend is going to be appearing at more and more events. School Christmas pageants, school graduations, eventually even your Grand Children's weddings.
You will have to make the difficult decision whether to be there as well or not. I'm sure that it will be a tough one for you to make. I'm glad that it isn't happening to me, because while it's easy for me to make comments on here, I know just how tough it is to make decisions in real life and you have some tough decisions to make.
The only bit of advice that I can perhaps offer and even then I don't know how it would work out, is to ask your ex daughter-in-law to have her new guy not talk to you if you should accidentally meet and say that at this point you would prefer not to even be introduced. Take Care. paul m