It is an 18+ manhwa. The ML is white haired and the FL is brown haired. They meet on a train where FL thinks she is alone but the ML is there watching her. When they meet face to face, he gives her a card to the organization he is a part of, which is for satisfying sexual needs. It is a place where there are different rooms you can play in and there are different levels. The ML is a gold or platinum member, and has a really dark background. He is forced into prostitution by his stepmother and stepsister, and has to please old ladies so that the stepmother and stepsister get money from them. He loves the FL and I think in the end they somehow get together and be happy.

Please help me find this manhwa. I think it's really popular.

An Innocent Sin

Thank you so much!