Fornication and adultery are synonymous, but they're not identical. Both cases are illicit sex, but while adultery is a blasé attitude involving the idea that sex is part of life, and that everyone who is an adult knows that, fornication is a specific instance of adultery, think "Lot pitched his tents towards Sodom." The difference is that adultery is always profligacy, having sex with someone you aren't married to, wherever either one of you is married otherwise or not, and as most people look at it it's serial promiscuity. Fornication is also called miscegenation and might be part of a legally valid marriage. In other words, thinking over the real difference, there can be monogamous fornication, at least in theory.
This one is kind of like the homosexual marriage, yes or no argument.