Threesomes are like souffles: it seems awesome on paper, but are really, really hard to make work and are often awkward from beginning to end.
Your boyfriend is probably getting his impression of how they work and how "pleasurable" they would be for you from porn, but just like real sex is vastly different from porn sex, porn threesomes only resemble real threesomes in that three people are involved. It's basically like real sex except you each have to communicate with two people instead of just one, and that's as difficult as it sounds. And if a couple is doing it, there needs to be serious discussions about possible emotional difficulties: most guys think they'll be fine with their girlfriend sleeping with another guy if they're there for it, and learn the hard way that this is decidedly not the case.
I'm guessing you're not into it, so just tell your boyfriend you're not into two guys: you only want him. If he wants a threesome, there are plenty of videos on the Internet for that.
This sounds to me like it's possible that the boyfriend may have a cuckholding type fetish?
Eh, maybe, but that's pretty rare. From the description, thinking it's just like RedTube is more likely.