This cults view on adultery and fornication is really weird. Basically you can do anything you like with someone else apart from ejaculate into her vagina and even then you can ejaculate into her vagina but it has to be into a condom. Only if you ejaculate into someone else's vagina is it adultery and fornication.
Taken from their site.....
In case a man's wife turns aside in that she does commit an act of unfaithfulness against him, and another man actually lies down with her, and has an emission of semen, and it has been hidden from the eyes of her husband, and has remained undiscovered, and she for her part has defiled herself but there is no witness against her, and she herself has not been caught......And the priest must make her swear, and he must say to the woman: If no man has lain down with you and if while under your husband you have not turned aside in any uncleanness, be free of this bitter water that brings a curse. But you, in case you have turned aside while under you husband, and in case you have defiled yourself and some man has put in you his seminal emission, besides your husband... (Numbers 5:13-20).
So lying down involves an emission of semen. Certainly the emission of semen enters into the sin of adultery. The Jealousy test is all about semen, and so there is no penalty on the woman if there is no semen because she passes the test. Therefore she cannot be put to death therefore she is not an adulterer:
Now a man who commits adultery with another man's wife, is one who commits adultery with the wife of his fellowman. He should be put to death without fail, the adulterer and the adulteress as well (Leviticus 20:10).
So when a man lies down with a woman this includes his putting his semen into her.
Now in case a man lies down with a woman and has an emission of semen, when she is a maidservant designated for another man, and she has not in any way been redeemed nor has freedom been given her, punishment should take place. They should not be put to death because she was not set free (Leviticus 19:20).
So the emission of semen applies also to fornication, so it must apply in the same way as it does to adultery. So both fornication and adultery require an emission of semen, i.e. semen from the man entering the vagina of the woman. So protected sex cannot be either fornication or adultery, so long as the protection works. Furthermore sleeping with another man's slave girl or concubine, is not adultery, for if it was then the both of them would be put to death. But:
And he must bring his guilt offering to Jehovah to the entrance of the tent of meeting, a ram of guilt offering.
And the priest must make atonement for him with the ram of the guilt offering before Jehovah for his sin that he committed; and his sin that he committed must be forgiven him (Leviticus 19:21,22).
Now in case a man seduces a virgin who is not engaged, and he actually lies down with her, he is to obtain her without fail as his wife for the purchase price. If her father flatly refuses to give her to him, he is to pay over the money at the rate of purchase money for virgins (Exodus 22:16,17).
There must have been an emission of semen for a purchase to be necessary.
We take 'emission of semen' as being inside the vagina to defile the woman.
Later on on that page he explains that if you sleep with your brothers wife you are a homosexual. Yeah I don't quite understand that either.