The price of cheating on a spouse, usually encompassing costs like alimony, child support, utilities, housing, cell phones, food, and vehicles.
The tax is often paid to both the spouse & the piece of ass on the side, and is necessary to avoid a public scene of marital discontent involving wrecked cars, burned wardrobes, trashed houses, midnight shouting matches in the front yard, etc.
SteveBro: "Dude, paying this adultery tax is killing my paycheck."
DaveBro: "Really, Bro? Try missing a payment on that shit and we'll be reading about you in the newspaper!"