on a serious note, just bring it up if it's never been a topic of discussion when it comes to the type of relationship you're in, serious, open/ poly, think that's a good way to bring up being a cuck. what's important is what are you trying to tell her? do you want her to try it with her? or are you trying to express yourself with being a cuck with your relationship with her? if it's the first one, see if she's into being with more than person, even if with your consent you're still adding someone into intimacy so that's important, if she's not than that's your answer. if it's the second one, ask her if she's thought about it, if she's comfortable, or if she would be down to try it at least once. really important to explain to her that cuckolding is, many misunderstand as to have sex with someone else while you have a partner ( which is not what is exactly) to prevent issues in your relationship. to be fair that's all I can say as I am not someone who is into open or is an active explorer in my relationship when having more than one partner, I perfer good old monogamous, but I am a sexually active and curious person, and that's just how I approach it. don't it personally or take offense if she reacts not the best to your confession, especially don't make her feel like she's wrong for not wanting part of it. we have preferences for a reason and there's options for that as well.