Christians in attempts at producing chastity in young people have wrongly taught sex prior to a wedding to be sin. This is one of the biggest messes Christianity has produced in young people.
Monogamous sex between a single man and single woman is not sin when neither of the two parties is lying to the other to get sex (that is, professing love merely to get sex). God does say however that He cannot have intimate relationship with a woman who engages in multiple sexual relationships because self control is one of God’s attributes. And let’s face it, it is difficult to combine self control with multiple sexual relationships. You about said as much in this write up.
Rejecting God because those who professed to know Him really were ignorant and did not properly represent Him is never a good idea. If the idea of the Christian God has merit, which I personally believe it does, you can choose not to participate in church while you get yourself together. I encourage you, however, not to reject God simply because Christians around you have or continue to mess up. We all are imperfect people trying our best to be the best versions of ourselves. Ignorance of course makes things worse than they need to be.
All the best as you attempt to arrive at the best version of yourself.