I meant, don't bring it up in MC unless you are SURE the MC's position is going to be that there can't be a third person in the marriage. Sounds like they'll support you in that, which is great, so yes -- bring it up.
I would suggest you immediately stop paying for his cellphone and his internet access out of joint funds. In fact, I'd suggest that you begin to deposit your paycheck into a new account, that's only accessible by you, and ONLY transfer funds to your joint account in an amount necessary to cover the family's NEEDS, and NOT his affair or his wants.
Let him know after you've done this; don't hide from it. Tell him "I have decided that I will no longer financially help enable your affair."
Do this with any items that he is squandering family assets on. He will have a new cellphone account up and running within hours, but you will have established an important boundary
