I usually go with a joke “My partner( he she or pronoun) fell in love and is marrying First Name. I don’t suppose he knew a First New Last Name, otherwise he would have gone for her.”

Or if they’re really rude I go for shock value:

“ well that is his mom and I don’t want my husband to fuck his own mom and I’m pretty sure his mom doesn’t want to fuck him, so once our sex life dies out in twenty years maybe it’ll be time to change it”

Or

“I’m too lazy to plan a wedding and a new identity that completely erases my own identity outside of him owning me. Ugh you’re so right. I’ll change my name and pay him money debts as he is now my feudal lord and I must serve his manor. While we are taking this time now is there anything else about me I should change when I’m married? New boobs? Different teeth??? Let me know and I’ll put it on the registry”