This is weird... Right?

My husband's mom is married to a wonderful man, and, his son is married to my husband's ex... so my husband's ex is now his step sister-in-law (I think that's how that goes). My husband and his ex dated for less than a year, but were on/off and remain somewhat in contact. At first, he felt weird about the situation, but overtime he has grown to not care much about it. I, on the other hand, still hate this situation, but it has been ok to deal with thus far because we haven't had any blended family gatherings ...until now. My husband and I have been together for some time now and my in-laws felt like it was time to put this weirdness behind us and do a blended family gathering. I have told them how I felt about this and that I didn't want to hang out with my husband's ex (I would feel awkward/uncomfortable and I wouldn't enjoy myself. I can't pretend I'm having fun.) They weren't very happy about that. They proceeded to have a blended family gathering, which I wasn't comfortable attending, so we missed out and my kids missed out. I have always felt that when a relationship is over, it's over. There is no reason to stay in touch with ex's unless you're thinking about getting back together, or you have children together. I understand this situation was not our choice, but am I being ridiculous for feeling this way? For not wanting to pretend like we're all a big happy blended family? For feeling weird about having my kids play with my husband's ex-girlfriend's kids?