Everything is permissible. To any normal person, this statement sounds like a "license to sin". And many have accused Paul of taking the grace of God way too far. But Paul answered that allegation in his letter to the Romans. 

When asked, "Should we go on sinning so that grace will abound?", Paul answered "By no means! We are those who have died to sin; how can we live in it any longer?" (Romans 6:2). To the Corinthians, he is filling in the detail as to why.

Yes, we are free to choose, but not all the available choices are beneficial. Some as a matter of fact lead back to bondage. They rob us of experiencing the freedom Christ won for us. Choose sin and you will find that sin grabbing control of your life. Choose the popularity of the crowd and soon you will find the crowd controlling you. For Paul, once he tasted genuine freedom, he resolved never to be mastered by anything or anyone other than Jesus Christ.

Here is the good news. You have been made alive in Christ so that you can walk in the newness of life led and guided by God's Holy Spirit. This is ultimate freedom - the freedom to live out who you are by grace, through faith in Jesus.

You don't have to "keep in step with the Spirit." But, when you give in to the desires of the flesh you put yourself under the control of sin. For the child of God, this doesn't make sense. Jesus died and rose again to set you free from sin and death. So, why give yourself over to something that wants to dominate and ruin your life?

Paul didn't want to be mastered by anything. He had had enough of that as a lost person. He wanted to live his life under the control of the Holy Spirit and experience love, joy, peace, patience, kindness, goodness, gentleness, faithfulness and self-control. That's what is most beneficial for you.