It's spelled out for you. Yes, we're saved by grace. But we don't get to continue to live in sin "that grace may abound." Being a Christian means continuing to live as sin free as possible. Baptism washes away past sins but we must be vigilant in our lives to avoid sin. Those who choose to live in adultery after becoming Christians, or be thieves, or murderers or any number of things, and refuse to give that up are endangering their soul's salvation.
It's not about who is right here as if it's a contest. It's about obeying God's commandments and submitting to his will. Knowledge of which is given to us by the Holy Spirit through the Holy Scriptures. If we refuse to obey we do so at our own peril. And if you look at it, God isn't being mean, or some such. Certain things are considered sin because they hurt us, they hurt others, and they hurt society in general. Our best life comes from trying our best to not sin. And asking for forgiveness when we do.