I think it is the modern 20th century fire-and-brimstone christians who find the Old Testament so appealing and have elevated to equal status with the NT.Well Jesus upon granting forgiveness would tell them to go forth and sin no more.
Sin was defined already so no real question about that.
But you DO have to repent your sins to obtain your salvation.
Historically, Roman Catholics, Eastern Orthodox, and most mainstream Protestant considered the NT to be a new covenant, superceding the OT.
To live the authentic Christian life means to emulate the Jesus of the New Testament. That was exactly the origin of the monastic movement, and they did not think sex with anyone was a good idea for the ascetic Christian life.
"The Christian Old Testament is, of course, the Jewish scripture, sometimes called the Hebrew Bible. In many Christian communities today, the Old Testament occupies a rather ambiguous position. Rarely are sermons preached on the Old Testament, and many Christians see it as irrelevant."
-- Bart Ehrman, Professor of Theology, University of North Carolina