James 4:17 Whoever knows the good and does not do it, to him it is sin.
You must know it iis a wrong thing to do before God says it is sin.,
is doing something you know is wrong
It is not doing something you do not know is wrong
So when you know something is wrong then doing it is Sin.
All sins are against God and are a rebellion against His Law, His Law of Love because if you Loved Him and expressed His quality of Love in all you do you would not sin.
Behind every sin is lack of love and a failure in stewardship either in a spiritual or physical thing. This is why the first sin by Adam was a lack of proper stewardship of Eve and the forbidden fruit in the garden. That is why Adam is said to do the first sin even though Eve appeared to be the first sin because she ate the forbidden fruit hers was actually the second sin.
Sin is a desire to be like God, to make your own decisions as to what is good or evil and occurs when this wrong desire is acted upon in your heart but not necessarily in the physical. You become your own idol, worshipping what you think in preference to what God requires of you.
Thinking about doing it is not a problem as you are fighting not to do it. The moment you decide in your heart to do it you sin even though you have carried out no action to make the sin happen in the physical.
Any deliberate disobedience, either moral or legal (in other words against the moral laws of God or the obedience He requires of you) is a rebellion against the authority of God. This is why Satan works on our desires to cause us to sin as it is wrong desires actaed upon that gives rise to sin .
Note: If you do not know it is a sin then you do not commit one as you are not deliberately disobeying the command not to do what you are doing. But the moment you know doing or thinking the act is a sin and you continue doing it then you are committing a sin.
As long as you are wholeheartedly trying not to do the sin and repent when you do it then God is satisfied as your attitude is correct. God does not examine results but does reward them. He examines the heart attitudes you had while you were doing the action or sin.
Behind every sin is incorrect stewardship which is why the first sin was committed by Adam who did not correctly look after the garden and eve.
Any sin not repented of will send you to hell
Common ones are:
Deliberately disobeying the laws of the land
Not wanting to reconcile with someone you have offended when you can do it (this is because you have not forgiven them unless because it is dangerous to attempt the reconciliation). Remember you must still desire too reconcile even though you cannot.
Anger that is not for God, His things and His Kingdom (a rebellion against His Love).
God looks at the attitudes and not the result. So as long as you wholeheartedly try to obey Him (so that you are not rebelling against Him) it does not matter if you fail because your attitude is correct and Calvary has dealt with the sin at the Cross.
Can they send you to hell?
Sin is deliberate rebellion against God and His Laws so if you did not know it was wrong then it is not considered rebellion. So sins done without knowledge they are sin do not send you to hell unless you keep doing them once you know they are wrong.
This is because God is merciful and not a Legalist and His Grace allows Him to overlook sins not deliberately committed against Him.1
It is all a matter of attitude
God has a principle that you are rewarded by Him for what you do and are punished for not doing what you are suppose to do (Gal 6:7-8). It is a rule of His and He is bound by it or He would be a hypocrite.
The punishment for rejecting His will and not repenting is to be excluded from His Kingdom, not obtain His blessings and allow Satan to rule you for his purposes until you repent. You may suffer physical problems because of your rebellion (sin) but if you repent you will get to heaven because the spiritual effects of the rebellion were removed at Calvary and your relationship with Him is restored unless you die in your rebellious state.
So a single woman who has a baby can have the relationship with God restored if she repents but she still has the physical consequences of her sin of immorality.
God does not send people to hell. They go there because they have rejected Him and His Kingdom and hell is the only place left for them to go to.