I think there is a lot of overlap but I don't think every guy who is into cuck porn or wants to be cucked IRL is bisexual.
For some, yeah it's an outlet to explore their sexuality and potentially seeking to please multiple people at once, even indirectly.
For others, they know or just believe they can't properly sate a partner so will step aside or make room for another who can.
And that's not talking about the degrees of voyeuristic and/or humiliation kinks that cuck porn can appeal to. So while every man who wants to be cucked or identifies with a cuck in porn, isn't necessarily bisexual or have any particular attraction to the bull. For a fair chunk of them, it's more about the taboo than anything else.