Do you think it might be bothering you because you know he's sexting her? Or that you know you wouldn't be doing the same thing if he was in your place? I ask because sometimes complete and total transparency can be a detriment depending on the situation. Like here, you feel that he should be 100% focused on you and he can't be if he's sexting with someone else, specifically a woman he hasn't established a relationship with.
You have enough on your plate already but is it possible that this is particularly bothersome because of the surgery up ahead? Things maybe feeling more intense than usual?