According to scripture, consensual sex = getting married with no possibility of divorce. It is also fornication. Something the religious leaders threw in Jesus face. "We were not born of fornication."
Adultery is another can of worms. Our modern "Christian" definition is not what the O.T. definition was. David had at least 24 wives (Possibly as high as the upper 30's) and had kids with all of them. God says he was guilty of adultery precisely once. When he took the wife of another man still living.
While Solomon took almost a thousand wives and concubines and had children with them, guess how many times he committed adultery. If you guessed 999 times, then you aren't using God's definition but modern man's definition. According to scripture he never committed adultery. He did take foreign wives though and God did reprove him for that sin.
Scenario for you... a man has a wife. His brother has a wife. His brother dies without an heir. What does God command the other brother to do? God commands him to raise up an heir for his brother, married or not. If he has a child with her before he has a child with his own wife that child becomes the heir for both brothers. That is the situation Ruth was put in.
Definitions for pornea/fornication: Fornication is bestiality, premarital sex, adultery and homosexuality.
No it's not watching dirty movies or seeing dirty pictures. That's our modern English word we created much later on. We can't shove that meaning back into the bible any more than I can ask how big an air hose the Holy Spirit needs to function or how big the Holy Spirit's compressor is.
The word for spirit is pneuma (Also translated air.) We created our modern word pneumatic (Air powered) from that Greek word. Like I said, we can't shove modern English meanings back into bible words. it doesn't work.
One more. There is a passage that says that if a man looks (Blepo: To dwell intently on.) on a woman for the purpose of dwelling on his lust, he has committed adultery with her in his heart.
Issues. 1.) God chose the only word available in Greek for married woman. 2.) God didn't choose the word for fornication, but rather a word that REQUIRES that a woman married to another man is the one present in the room. Adultery.
All of these things listed in this passage are pointing out the prerequisites for the actual sin is where we need to stop. For example: Don't get angry and you won't murder someone, and don't sit in the same room with a woman married to another man and dwell on lust and you won't commit adultery ... Those are the meanings, nothing more and nothing less. Everyone that wants to change the meanings into something much more broad has a private doctrinal ax to grind and he is butchering the meaning of God's word to do it.
Problems arise when we don't stick to the meaning of the words and the context. Those that would create much more out of the previous verse will have a cow when we look at the next passage.
1Co 7:1 Now concerning the things about which you wrote, it is good for a man not to touch a woman. 2 But because of fornication(Includes the premarital sex concept), each man is to have his own wife, and each woman is to have her own husband.
So, from the context, what kind of touching is going on that leads to fornication. Is that "touching" a sin? (Sorry. Trick question. According to the scriptures, whatever is not from faith is sin, so if you believe eating turnips is a sin it is for you.) Let me rephrase that question: If you are touching in such a way that there is an increasing danger that you will become engaged in premarital sex are you already sinning according to scripture? Is that what the bible says and means or just your "feelings"? Yes, until your "feelings" change you are under the whatever is not from faith is sin category.
Here is the standard for determining if you should get married in the N.T.. Do you have self-control or not? Are you in danger of committing fornication or not?
1Co 7:9 But if they do not have self-control, let them marry; for it is better to marry than to burn with lust.
Remember, we are talking about needing to get married to prevent fornication because you both don't have self control... because you are touching ...
Have you sinned? Yes, according to scripture.
1Co 7:28 But if you marry, you have not sinned; and if a virgin marries, she has not sinned. Yet such will have trouble in this life, and I am trying to spare you.
Want it a little clearer? Context and flow of thought, meaning of words as they are used throughout scripture and in their context here confirms the interpretation. Yes, things are getting out of hand, er... into hand, and you are treading dangerous waters. You get married before having intercourse. Have you sinned in the eyes of God?
1Co 7:36 But if any man thinks that he is acting unbecomingly toward his virgin daughter, if she is past her youth, and if it must be so, let him do what he wishes, he does not sin; let her marry.
Yes I know you will probably never see that in any "Christian" church sermon. It's a shame really. The other issue is defrauding, but I have to leave some areas for you to research on your own. So the question is, if you can have things deteriorating to the point where you have to get married to prevent pornea/fornication do you really believe that being in the same room with a single man or woman and dwelling on lust is going to be a sin? We are well past that point here people.
Lastly, further on in the N.T. God broadens the definition of adultery to the destruction of an existing marriage relationship by committing adultery (Fornication resulted in another wife) or by divorce to form another familial unit. (Familial unit = Husband, Wife/wives and all children)