Romans 1:32

Once again Paul repeats that the people he is referring to know that what they are doing is wrong and know the appropriate punishment for it. We are not held responsible for something we don't know; God has put the understanding in our heart and so we have no excuse if we choose not to obey Him.

Interestingly, he then takes it a step further: it's bad enough to commit the sin yourself but how much worse to encourage others to commit it also. Like Matthew 18:6, causing or encouraging someone else in their sin is a sin itself, just as bad or worse than committing the sin yourself. Jesus raised the standard higher and Paul is communicating Jesus' teaching. In Matthew 5:21-32 Jesus put the sin as not just committing the actual sin but even considering it or imagining it. Encouraging sin in someone else would be pretty similar: technically not committing the sin but only because of circumstances. We can see this resonates with our nature when we consider a specific sin, such as murder, and the guilt of a person who pays someone else to commit the actual murder. They are guilty of the sin even if they weren't the one acting it out because they are the will desiring the sin to happen and the encouragement for someone else to make it happen.