I don't think our sexual orientation defines who we are, either. And I firmly believe it's no more relevant in the assessment someone's honesty than their IQ score is.
Gay people who enter a heterosexual marriage do so for many reasons, not the least of which is the ability to share their life with their chosen partner and raise their own biological children together. Gay, bi, or heterosexual marriage is about a LOT more than sex. And we all role play in the way we present ourselves to the world.