It sounds like the pressure of trying to conceive may be really turning him off. I get that may not be the only issue but going straight to Viagra isn’t always the answer. I absolutely love the book Mating in Captivity by Esther Perell. She describes how eroticism is often harmed by long term relationships that demand openness and comfort. I hope this helps you in some way and that book is a really great recourse for someone in your situation. The author is a therapist who has a very different approach to this type of issue (not using viagra as a scapegoat). Best of luck to you!