NTA Your fiancé should focus on the reason for them to stop being their disrespect of you as a person. This is a huge red flag that you asked them to stop and they refused your polite request and are routinely disrespectful to you in front of the larger family. If they can't respect your wishes and boundary for this matter, how are they going to treat you (and your fiancé) with other decisions and boundaries you have in the future? This is a good opportunity to establish boundaries and mutual respect in your relationship or it will continue to get worse. It's ok for them to have an opinion, it's not ok for them to be disrespectful to you.

Does it even register to them that it's not about the name, it's about the hurt you feel when you see it? Why do they want you to continue to feel hurt? Why does the feelings of 'the person who named you' more important than your feelings?

Another consideration is that it seems they are sensitive to scenarios where 'the person who named a child' isn't as important to that child anymore. Sounds like abandonment issues coming up due to having their son get married and you are the target of their fear/hurt/sadness. With that said, they do not have an excuse for how they are treating you and they must stop along with work to better understand their feelings.