Does the 6th Commandment only forbid adultery?

I have a friend who was raised Catholic but is now an Athiest. He said the sixth commandment does not forbid doing ‘it’ before marriage, but only restricts it to one spouse ONCE you are married, and goes on to say that that is the definition of “adultery”. I know he is wrong but I am not sure how to argue this in an effective way. I was not able to draw on any points in the bible to back up my point, but did talk about the logic in how a family structure protects the material, emotional and spiritual welfare of children, but it came out sounding to him like an argument in the cultural sense instead of moral sense.

How would you respond to this if someone said this to you?

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If it doesn’t violate a commandment how can it be a mortal sin?

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