Hi OP. It sounds as if your DH is bisexual, but didn't know it when you married, or wasn't comfortable with it, and hoped by marrying you, that that side of things would go quiet, or go away. It's easy to see what's happened within your marriage as betrayal, and it would be reasonable for anyone to consider it as such, but depending on how good other aspects of your relationship are, and how much you like and love him, you might be able to repair things and have a future together.
Sexuality is a powerful force, and one which is largely biologically determined, at least for most people. There might be some people who have chosen to be gay, but most will say there isn't a choice. It is what it is. For heterosexual and homosexual people who choose to live in monogamous relationship, at least there's a good chance that some or most of the sexual needs and desires within the partnership can be mutually catered for.
The same can't be said for bisexual people, because there's no chance of having their sexual needs and desires met within a monogamous relationship, to the same degree as for heterosexual or homosexual people. Some might argue that repression of desire is required in such cases, but sexuality isn't easily contained and controlled in this way.
Also, full expression of one's sexuality can often bring a lot of shame with it, which can be amplified within certain cultures and faiths. Add to this the problems with prejudice and discrimination etc. All these kinds of things are bound to affect people's ability to be honest with themselves, let alone being honest to anyone else.
So I guess the point is whether your DH is at a point in his life to face up to his true sexuality and be honest with you about his past and whether he wants and seeks forgiveness from you for the pain he's caused you and by extension, his family. You would then at least have the choice of deciding whether you want to move forward with the relationship, setting new boundaries. Obviously, this is unlikely to ever be an easy choice.
On the other hand, if he is in fact gay, then to give yourself the best chance of finding a fully loving relationship with someone else, you should leave as soon as you can.