You’re not overreacting. This is an emotional affair, and while only half of emotional affairs end in something physical, the bottom line is there is a betrayal of intimacy. The fact that your partner doesn’t honor your feelings with this, chooses to continue even though it upsets you, and it has escalated all point to the fact you must take this very seriously. His trying to minimize it and saying that you are irrational doesn’t change the fact. It upsets you and he continues to do it anyway. I strongly recommend couples counseling over this. Keep the issue on the front burner. In my experience and in the experience of many therapists, an emotional affair can be easily as devastating as a physical one.