God is not a fornicator, but faithful in all that He says and does. To transgress is to break all the commandments of God at the same time, but I just show you a few:
Exo 20:7 Thou shalt not take the name of the LORD thy God in vain; for the LORD will not hold him guiltless that taketh his name in vain.
Committing fornication is to misrepresent God's own name/character.
Exo 20:12 Honour thy father and thy mother: that thy days may be long upon the land which the LORD thy God giveth thee.
Committing fornication is to dishonour ones father and mother, to whom they belong in God's stwewarship, until one is married.
Exo 20:13 Thou shalt not kill.
Committing fornication brings death, since it is sin, but even more immediately, since the person or person that are committing fornication do not have the blessing of God, neither of the Father and Mother in wedlock, the persons bodies themselves, built by God (Exodus 20:11), will actively fight off the other persons organism, as the body sees it as a threat. It's why there are many STD's, and autoimmune diseases in the world. Much of it is the body's reaction and defense against illegal activity. In
true marriage, God unites the two minds and hearts together, and so too their bodies come into harmony with one another, no longer seeing each other as a threat at the cellular level. That is more of a medical study though and I briefly mention it.
Exo 20:14 Thou shalt not commit adultery.
"adultery" is not merely physical, since all of the law of God is "spiritual" (Romans 7:14), and not merely addressing the physically married. It means, breaking of faithfulness, going back on the word of the vow (apostasy), see Ezekiel 16:38, 23:37.
Exo 20:15 Thou shalt not steal.
Committing fornication is to "steal" that which does not belong to you, in that the person with whom the fornication is being committed is unmarried to you, and not given to you by their father, their mother, and most importantly by God.