If you believe in God, you must agree that He makes the rules and sin is what He says it is. Otherwise, you really only believe in the idea of God.
And if you really only believe in the idea of God, then you have no way of defining the difference between good and bad with any sort of intellectual integrity – it is whatever makes you feel good or bad, whatever you do or do not want, whatever makes you feel comfortable or uncomfortable.
If you decide that you have the right to make those judgments, then other people have the same right. You’re not entitled to deprive anyone else of making their own judgments. You also have no right to demand that their definitions have to make you feel comfortable.
In short, either we have one God making the call and setting an absolute standard, or there are no absolute standards.
And if there are no absolute standards, then everyone’s standards are equal. Our “standards,” by definition, will be self-centered, arbitrary, and generally very convenient.
When you talk about what God does or doesn’t accept – unless you have something He has given in writing to back it up – what you are really saying is, “this is what I believe. These are my judgments and these are my laws.” You are determining what is right and wrong, in essence, making yourself God.