Lots of good answers so far. Do you want to stay with your husband, work it out? Isn't this considered the for better or worse part of his wedding vows? Why does he need to seek sexual intimacy with another outside of his wife? I wouldn't allow that, and if he went ahead regardless I would separated until he finds some loyalty and compassion. Better to find a solutiion together then him doing that. Worse too you say he makes you feel guilty.
“The world would be a nicer place if everyone had the ability to love as unconditionally as a dog.”