This is a huge issue, it is for him, and he is wrong if he thinks it shouldn't be as big an issue for you. You are NOT a lesbian (I assume) and if this is something he has to do it IS going to change your relationship. You may still love him for the person that he is but face it, marriage is more than just love, there is a physical aspect as well, and he can't expect to make such a dramatic change and have you just follow along. I hope he understands that.

Even if he thinks otherwise now, he obviously still has issues, whether he is still considering that possibility or if he is truly past it and dealing with PTSD problems. Those are real issues that will effect your marriage too.

Given that you don't really know what is behind his current behavior I don't think it is strange at all for you to be questioning the "lesbian" issue. I think the point is that something is wrong, and you have to question why, and since that is the only thing on the table it is natural to question that.

Doing therapy together is a great idea. If there is a solution to this I think the answer would be there, and with his own private therapy. When his issues and your issues together are on the mend and you both understand them then I think you can begin to "put it behind you". I think the point is that you can't put the "lesbian" thing behind you, you have to put the whole package behind you. And that is possible. If it is something that you just can't get past for whatever reason then that is a question you will have to face together... but you have to understand and try first before considering that.

Good luck.