If your not into it either ignore or say no. Do not behave like a child and respond with an obsenity just because his tastes may differ from your own. Why does this make him an arse?
As for what the word cuckold means?
Traditionally it refers to a man who has a wife that has cheated on him.
Now it usually refers to a man who is in a relationship with a woman who is free to have sex with another man, while he stays faithful. The cuckold is o.k with this....
My DH is a cuckold, and it works very well for us. 