God? Or man?
Of course it is, that's pretty obvious. It's called fornication, lust, sexual immorality. God designed sex to be kept within the confinements of the marriage bed. God calls people who commit this sin Whores and Harlots.
Given this was before the printing press and most people were too irresponsible to take on a complex life, living the simple life made simple sense.
That's why the single book that man passed around to be the most well-read preached this simple life, which worked simply and best.
Even in our day'n age today, far too many people are still
too immature to take on the responsibility that comes with alcohol consumption, gun ownership and sexual intercourse.
If you are too immature to do so, then you should consider following a simple life, like that one is in the Bible or other, similar text.
If you cannot limit your lust for women to the point it does not harm you, your view of women or even the woman herself, then it's a deadly sin.
If it's one you entertain only for individual pleasure and one that does not dominate or skew your ability to respect the consent of others, then it is not.
I've had this argument in many threads, including Erica Rose Campbell's more recently.
It explains a lot of what is wrong with society, and why some people (even many people here) "need religion" or some "simple rules" in their own lives.
Because they cannot see the obviousnesses of the truth on valuing others, always seeking consent, not excusing things but understanding that the values of others are more important their yours when it involves them, and how irresponsibility is what leads to everything that is wrong in this world.
If you can master that balance, no matter what it all entails, then things are not a sin for yourself, much less not a sin by you with regards to the people around you.