Well it is true that if you decide to come out the gender of your partner will attract attention. What would you expect/plan to happen to your relationship with him if you were to come out?

If you continue to be in a relationship with him, I suspect that people of all orientations will consider you to be unavailable, whether or not they believe that you are gay.

I am guessing that the statement "I am gay but right now I am in a heterosexual relationship" has been said before. Maybe you could call a lesbian/gay hotline and talk things over anonymously with someone who can appreciate the complexity of your situation.