I did that badly at first, and sometimes still do. My WS was in a hotel with the coward OM, and every hotel in town we drive by I wonder.
Did they make out in our family car? What kind of sex, how long, and it only gets more graphic. Same as you asked, did she fellate him then kiss me? For a solid week I drove myself crazy with this.
I do try to reason it like somanyyears suggests and it works better as strong emotions simmer down.
After only 2 months for me it's better, though.
I'd recommend that even if you try to work out your marriage in some ways you emotionally disconnect from him and these activities. I've had to do it to keep myself sane. In some ways it makes my WS upset and makes her feel she's losing me, which she is.