Hi Wife,
None of us can answer whether or not your husband is gay...or will identify as gay in 1,5,10 years. But, if he wants to have sex with a man (as you say he does) he's not straight. Are you OK with that? If not, it's OK to say: This isn't what I signed up for. It's OK to insist on monogamy. You can still respect his identify without agreeing to anything you're uncomfortable with.

Wife669: "Is it okay" covers a range of hidden agendas, from "is there something wrong with me feeling this way" to "are you going to be mad at me if I admit I've known this a long time and even have acted on it".

There's also "is it okay with YOU" which is a different question -- one Ellexoh and I are both asking.  Is this the relationship you signed up for?

And to all who commented on the gay/bi thing, I will say that my own x used up that delusional excuse and I'm afraid I have about zero ability to accept it.  I get that some men and women are bi, and those people should be honest with their partners from day one and also should understand that "bi" doesn't translate into "allowed to cheat".  You enter into a monogamous relationship, and induce the other person to enter into that relationship with you, don't tell me ten years in you accidentally forgot to mention you're bi.