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.