Husband set me up for a foursome

My husband had always insisted that we go for a sleepover at his best friend’s place. We recently went and I retired to bed earlier than everyone. I woke up later and found him making love to his friend’s wife in the presence of her husband. I was so traumatised since it appeared to have been going on for long and I don’t know what to do. I think the plan was to have a foursome but I am not the kind to do that and my husband knows it. Is it unreasonable for me to want to leave him ? I also wonder if his friend and wife consider this as infidelity. Please advice me. I am stressed. {Anne}