"Cuck" can be an insult, but it isn't inherently one. A lot of people I know who do this sort of thing though get off on the humiliation aspect of essentially being emasculated by another man, so they would enjoy getting called that.

I should note all my friends are adults over the age of eighteen. I do not know your age, but if you are a minor it might be harder to engage in this sort of activity. Personally, I came into a group through a friend of mine. I believe other people in this group found it through word of mouth and through adult websites. However, again, you have to be an adult to engage in these sorts of groups (though the website is 18+, the group is 21+).

I think the best way to bring it up is to be direct. Hopefully you and your girlfriend are pretty open about your sex life with each other. Let her know this is an interest of yours and ask how she feels about it. It can't hurt.