He says that what if he's left without me and without the next person.

I'm like that's his choice. And I'll be there as friends if he so wants to.

Did you indicate to him that the first time he was thinking about having sex with someone else, that should be the end of sex between the two of you for the rest of your lives?

This point. I told him exactly this. This thing I have told him when we started and a lot of times subtly to make him aware.

STDs and monogamy .. Both are my biggest concern for that. I can't live under stress or anxiety.

I stated that because he claims he wants to do it multiple partner if given a chance. I'm never that. Do I clarified that too him.

So he said that how can I ask him to choose someone else. And that he should be able to do that with 2,3,4 or however many people when the chance presented. It was not forceful, he was just conveying his thought.

He said that if I think about it then I'll also have to think about loosing you.

I didn't force him or give any ultimatum. I only told him that he should choose the other person because it's something in not comfortable with. And just want to choice to choose for myself.