I am going to agree with skeezyks here. The only person you can change and have control over is you.
He has made it clear he has no interest in changing so the question is can you live with it.
It might come from a deep rooted view of his sexuality, but really the why matters very little.
What you choose to do from here on in does. If you choose to stay after making feelings known, and he refuses to change then you will have,to bite your tongue and live with it.

Whatever you decide, I wish you all the best.