Baby making has ruined our lovemaking

My wife is desperate for a second baby, but I can’t keep my erection and as a result am not ejaculating. I have tried Viagra but it has not worked. My wife phones me up at work to come home when she is ovulating and I am beginning to feel like I am just a baby making machine. Our first child is now seven years old and we have not had a sex life since he was born. His birth left me in shock because it seemed like everything that could went wrong. It was all very traumatic, worrying for my wife who wasn’t coping and worrying for the baby who we nearly lost. But she seems to have forgotten all that and cries at the thought of not having another child, whereas I would be happy either way. I don’t feel I could ever be honest with her about my true feelings about getting pregnant again. I feel trapped and wondered if you might have any advice?