In terms of sexual fantasies I'd say it's because there's quite often a progression. For example, Amnon was attracted to his half sister Tamar and went on to rape her. David fell in love with Bathsheba, which then led to the death of her husband who he purposely sent to the front line of battle. Lusting after someone else's husband or wife could potentially result in an actual affair. Similarly, people who read explicit literature might be desensitised and move to watching pornography, which in turn causes further desensitisation and seeking out more and more depraved images for the same kicks. If married, it could also cause the spouse to be dissatisfied with their husband or wife, create conflict and eventually end in divorce.
There's an awful lot of 'could' and 'might' in there, and an awful lot of people who seem to have no will or self control. The transgression happens when the lusting turns into an affair or a rape. And that is the responsibility of the individual.
There's no evidence that explicit literature is the gateway drug to extreme porn, unless you can provide me with some - I read (and write) some reasonably spicy books, but have never felt the urge to pursue anything extreme. I don't have hangups about masturbation in relationships, though I know some people do, but if you put that aside, where is the harm for someone who is single? I mean, I haven't felt the urge to rape anyone or send them to war.
And when sex isn't working in a relationship and attempts to improve haven't worked, for whatever reason, divorce is a perfectly reasonable thing to pursue. Divorce in general is often the better outcome, and not just in cases where there is abuse.
What I'm seeing in your version of Christianity is all about suppression and self-denial. That's one method, but there are others. All involve taking personal responsibility. That can be done without faith being required.