The bible is very specific about adultery being a sin.
The Bible, the Law of Moses, and the current civil/criminal law of numerous countries and states, descended from various earlier sources including English Common Law, is very specific about the definition of what adultery is. Basically, it's coitus between a married woman and any man other than her husband. The end. We find the biblical prohibition and definition in Leviticus 18:20 "You shall not have intercourse with your neighbor's wife , to be defiled with her."
And more in depth, in Leviticus 20:10 “If a man commits adultery with the wife of his neighbor, both the adulterer and the adulteress shall surely be put to death."
The very word "adulterate" means to add a foreign substance; hence "adultery." And that foreign substance is strange seed. State v. Lash
"There never was an action for adultery known to be maintained at the common law by any but a husband showing that the offense can not possibly be committed with any other than a married woman. The heinousness of it consists in exposing an innocent husband to maintain another man's children and having them succeed to his inheritance. This is the common law doctrine of adultery transmitted to us from the earliest times by those venerable sages who gathered it from existing precedents, records, and decisions at the times they respectively wrote. I shall cite only a few of them, because the records and decisions referred to by them have been so faithfully consulted and the testimony of those sages examined and condensed with such admirable precision in the imperishable commentaries of Blackstone
that it is almost vanity to go behind his work.
"More definite language can not be selected for confining adultery to illicit intercourse with a married woman than his following definition of the offense: 'Adultery or criminal conversation with a man's wife.' The woman must not be single, she must be another man's wife, and whoever, married or single, has illicit intercourse with her becomes guilty of adultery. The text is in III Bl. Com.
139 and is so clear of ambiguity as to challenge any attempt to evade it."
So what we see is that woman must be married and it doesn't matter whether the man is married or not. It is adultery due to the marital status of the woman.
In Blanchflower v. Blanchflower
, NH Supreme Court ruled it was impossible for two women to commit adultery since intercourse was coitus.