Odds are he’ll do nothing at the pharmacy aside from being amused by this convo. If not, tell him the truth. Then ask him to co-write a buddy comedy screenplay based on this misunderstanding.

Making a big announcement would feel like deliberately stirring drama. But I see no reason not to drop it into conversations when it fits. For example, Guy Friend J rolls his eyes and mentions that your ex was flirting with his girlfriend again. You then have an opening to say “I wish he’d grow up and stop playing games. I found out a couple months ago that he sexted someone overseas for almost two years behind my back.”

How long has your SO been sexting? Have they put a lot of time/effort into hiding their activity from you? That would suggest they knew you would not be okay with it. Now that you’ve found out, have they invited you to sit in so that you can see how totally-not-cheating it is for them? I’m guessing that’s a “no.”

Some people feel that sexting doesn’t count as “real” sex, and that as long as they’re discreet and they never plan to meet the other person then there’s no harm done. Others strongly disagree. If you two haven’t previously discussed your opinions about things like sexting with an actual human on the other end, sexting with an AI, consuming pre-recorded porn etc, and you’re just now finding out that you’re not on the same page, you’re overdue to clear the air.