Godparent to be or not to be?

When my childhood friend and his girlfriend became pregnant they asked me to be the godfather of their baby. A few weeks ago, they announced that her brother would also be godfather to the baby. In the same conversation they tell me that because they are not allowed to have 2 godparents of the same sex, I was welcomed at the ceremony but as a witness only....I take this situation very seriously and because they have obviously chosen her brother as godfather and I respect their decision, I feel uncomfortable continuing on with the charade of being named an "honorary godfather" you either ask someone or you don't is how I feel. How do I respectfully go back to being the very happy friend of the expecting couple? I would rather just attend this ceremony, they seemed very uncomfortable in trying to explain to me why they asked me in the first place but then realized that I couldn't be. I would rather not cause friction if that is in any way possible!