Did you know you were gay when you married her? If so why did you marry her in the first place? If you still love her you should just let her down gently and apologize for not telling her sooner and say you don't want anything to happen to the kids and you still want to see them (if you do that is) but you should tell her soon.
I think the most mature thing to do in this situation, is to think about your family first. Do not be selfish. This does not effect only you-- you are the father of two children. It does not matter if you are not the sperm donor... being a father is an action. You should stay with your wife, wait until your children are 18 years old, and then you can change your situation.
Flip the situation::
Remember being a child? Remember needing/wanting a father?
