my SO is cheating?

If you're awhere of SKIBBEL you know it's a sexting starngers website. Now my SO swears to me he doesn't masterbate (which easily we know is a lie), but I came across him using skibbel.com which really upset me he's sexting other girls to get off when he has me. I don't care about porn, I watch it too, but online sexting is like cheating in my eyes. Am I wrong to be upset? How should I go about confronting him? And what would you do?