You have to ask yourself what it is that disturbs you about the idea, never mind the fact, that your boyfriend wants to suck another mans C***. I suspect a lot of men have this fantasy and I'm not convinced he is telling you the truth about being handcuffed to the bed and sucked off by a man - this might just be boasting.
However, if your worry is that he is gay, or even bi, then speak to him about it. Tell him why you are uncomfortable and see if you can agree either that he should do it or that he should not. As I've said, many men have this fantasy and of those I've discussed it with most said that they tried it once and it was a complete turn off. If you don't let him try then this will remain an issue between you and end up spoiling your relationship.
My husband admitted to me that he had always wanted to have a mutual
masturbation session with another man - this after 15 years of marriage. I wasn't too keen either but as I'd have same sex flings myself couldn't really complain.
I set him up with a gay friend and he really enjoyed it. And yes he did suck the other guy off although not on "the first date".
Their relationship lasted a few months and since then he has been strickly hetero (as far as I know). He says he enjoyed it but having done it the novelty wore off.
If you really think he wants to do it then try to find it in yourself to help him. Whether he likes it or not it shouldn't really cause you any problems and if he finds he prefers guys then kiss him goodbye now otherwise you are just delaying the inevitable.