Dealing with Infidelity in Active Addiction

I had a similar experience. Accept I was the one having the sex. My ex wanted to watch me with another man, I said yes at the time and grew to regret it after. I was on pills when it happened, substance makes you do things you wouldn't usually do without thinking about it first. It's okay to regret it, you should be able to speak to him about this regret. You should be allowed to express your emotions. No need for anger on his part, he may be confused, thinking you're angry at him for it rather than regretting the decision you personally made, maybe try a different approach at the conversation?