plus you can make an inference from the Sermon on the Mount.
27 "You have heard that it was said to those of old, "You shall not commit adultery.'
28 But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart.
Now, you can't have sex without lust, so having sex with anyone that isn't your spouse is basically adultery, inside or outside of marriage.