Part 1.
Because, if you are unfaithful to your spouse and embark on an adulterous affair, you will incur the wrath of Jehovah which will lead to you being disfellowshipped and ultimately being slain by his hand at armageddon.
Part 2.
You would never do such a thing because you love your partner too much.
..and therein lies the rub.
I've known many newbie-ex's who have played around once they've decided to leave the JW's. In many cases it was because they hadn't even thought about Part 2 above. Part 1 was such a dominating factor that when they left they abandoned that belief. Only in hindsight did they see that Part 2 was of much greater value.
It just never had a chance to grow.
Englishman.