How does Viagra work?

I'm with someone who wants to try Viagra so I bought some generic. He's taken Viagra twice to no avail. however, he takes it and makes no effort to make any moves. I told him I wish he'd just go to his Dr. and get the real deal but he doesn't want to. At the same time, now he wants me to go buy something sheer and sexy and THEN we'll have sex. My question is why should I have to do that when I already bought him the Viagra? I think he should go buy me something. But I am more hurt thinking he just doesn't want me the way I am. Am I being overly sensitive? He keeps talking about how we're "going to do it" someday but someday never comes. I want to be sensitive to his ED problems but I am starting to take all of this personally. Thoughts?