What should you do if you find out your wife is a lesbian?
This is really an opinion thing, but here goes. If she is truly a lesbian and is not just curious or bisexual, then I think you need a divorce, as this would make you incompatible on the most basic level. Can you see yourself married to a man? Also, I don't think you should be offended if she is a lesbian. I would not see it as her using you, she obviously did not want to "come out of the closet" and needed a husband to keep up her charade, and she loves you enough (though not as a husband) to select you for that job. Or she may not have known she was a lesbian at the time when you were married.
If she is bisexual or curious, then this means she is capable of loving you as a husband and your marriage is worth saving. If you don't want her to have sex with other women, then you need to express this to her. I would not recommend threesomes, however, as this almost always leads to jealousy and would only hurt your relationship even more.
You would decide, after long deep conversations if you two want to remain together despite the differences in sexual orientation. As Ask Abby says, "would you be better off staying together or being apart?"