It isn't private if it's affecting your sex life together, and your happiness. From your heartfelt post, it clearly is. For your own self-esteem if nothing else, you need to confront him about it. You can ask us and get all sorts of opinions and advice, but the person you really need to be talking to is your husband.
If you want to "catch" him at it, try to make it fun and include yourself. Say "Hmmm, mind if I help?" and just join him. Perhaps you can build a bridge from his habit back to you two as a couple. Mutual masturbation can be very erotic and satisfying for both parties. However, if you choose to confront him about it LATER, when he's NOT doing it, I would say "Honey, it hurts me to know you feel the need to look at pornography and relieve yourself every day when I'm right here almost any time you want me. I love you very much, and it makes me feel ________ when you do that. Do you understand?" ... and just let him talk.
