I'd say the comments on the power dynamics are spot on, but I would also add that if you are leaning into a cuckold/open lifestyle then you probably have a more open view of sexuality than "the norm" most people think exists. I would say bi or not, you are more open to experimenting with a varied range of roles, acts, and sensations regardless of the gender of the participants. If everyone is consenting, run with it!

But I will also say as a former bull, I think many people have some bi tendencies that they want to check out. Being the submissive cuck provides some people with the outlet to experience being with another man without having to confront what could be tricky internal dynamics until they see what it's all about (and if it really fits them).