He cheated, I forgave, and now eighteen months later he wants to open the marriage

Most of you will tell me to run. Hear me out anyway. Eighteen months ago he had a four-month thing with a colleague in Toronto. I found out by accident, the usual cliché — phone face up on the kitchen counter, message preview, blood drained out of my face. We did couples therapy with a guy in Yorkville for almost a year. It was brutal. He moved jobs. We rebuilt. Now he says he thinks the affair happened because he 'locked up a part of himself for fifteen years' and that he'd rather we open the relationship honestly than risk it happening again under pressure. He swears he doesn't have anyone in mind. He says he wants to do it together, scene first, talk first, never solo. Part of me knows he might be right. Another part of me thinks this is the exact sentence a man uses when he wants permission for the next one. A third part — and this one scares me most — is curious. Has anyone actually rebuilt after infidelity by opening up, and how did you tell whether it was healing or anesthesia?