I disagree. I watch porn, and all three of the women that I've had long-term relationships with watched porn with me on multiple occasions, always leading to really hot sex afterwards.
Most men look at porn at least once in a while and there is nothing abnormal about it, unless it's something disgusting and illegal involving minors or animals. Cheating is hooking up with a real, live person, not looking at pictures or videos of total strangers. If you're not willing to have sex with him anymore and you won't let him look at porn, then he may feel that he has no choice but to cheat on you with a real woman.
I understand your concerns, but it sounds like you're using your child as an excuse for letting your body go. You should start exercising. It might even out your emotional swings, boost your self-esteem and increase your attractiveness to your husband.
Or you can just lay down the law and waiting for the real cheating and/or inevitable divorce.
Good decisions come from experience. Experience comes from bad decisions.