I feel so bad for couples like you, I really do. Honestly though, if you find the conversation going in that direction, just be blunt and say "this topic is not up for discussion". Maybe you can find a nicer way to say it
I had a friend tell me how him and his wife have been trying for four years, they've both been checked out, they are both fertile. He was so frustrated, I didnt know what to say. So I didnt say anything. I just let him vent. I think it made him feel better. I havent seen him in a couple of years, I dont know if they ever got pregnant or not.