Partner keeps reopening Sniffies despite multiple "talks"

I really need some outside perspective because this is eating me up inside. My partner and I are approaching our 5-year anniversary this June. We’ve been through a lot—from long distance to now living together— and overall, things have been great. We’re deeply integrated into each other’s families and have built a real life together.

The Backstory: We met on Grindr years ago. Last year, we discussed potentially using Grindr or Sniffies for "extra fun" together, but we never followed through. Eventually, we agreed it wasn't for us and deleted our accounts.

A few months later, I found a Sniffies account in his phone history. I didn't confront him immediately; I monitored it for a bit. He was messaging guys and getting pictures, though he never sent any of himself or physically met up. When I finally confronted him, he apologized, claimed it was a mistake, and deleted the account in front of me. This has now happened three separate times. Yesterday, I had a gut feeling and checked the site again. Sure enough, I found his profile (based on stats and location). When I asked him about it, he admitted he was "horny and just wanted to see what was in the area." My response is: Why not just watch porn?

My dilemma: I believe him when he says he hasn't physically cheated, but it’s the repeated lying that is the problem. We are supposed to get married at the courthouse this June. He wants me to have a formal sit-down with his parents soon. I’m now second-guessing everything because he keeps promising to stop, only to go behind my back again weeks later. How do I move forward from here? Am I overreacting since he isn't "physically" meeting anyone, or is this a major red flag before marriage?