What you don't understand is the situation of a SEMI-HAPPY MARRIAGE.
You keep spouting this like it is some sort of holy mantra. You got that expression from books and websites that i drop on the "ignore-everything-else-feel-good-about-yourself". This is a category i mentally created for every piece of text that doesn't really help you in any way but can be used as self justification for whatever you want to do.
There are a lot of stuff that falls into this category. These days some psychologists create names for everything because we all know that labels make people feel better.
Typical situation i deal with everyday due to profession:
"You not horrible parents, it's just that your kid has [insert bulsh!t disorder here]"
"Mother" (biological meaning here)
"Thank you so much Dr. Quack, i can finally resume banging the neighbour with my son in the house when my husband goes for out for drugs."
Due to situations like this people miss the chance to improve themselves and makes things work. They just flee the situation and head for something that may well be worse (in the example, usually pumping the kid with drugs).
People who write these books are in it for themselves. They know that marriages can't be 100% happy all the time (many of the people who write this stuff have unstable marriages themselves, yet think nothing of offering advice). What they offer in exchange for your cash is a feel-good band-aid and some crap advice that can't be followed by normal people because we can't spend weeks dealing with the stuff. We have to work, take care of kids and we tire.
How many people have dream marriages when everything falls into place with no lows? I sure don't know any. Is everyone in a semi-happy marriage at some point in their marriages then? And it does beg the question. Are you sure you want to be married? Because, even though you don't want to hear this, there will be a time when things will not be so dreamy with that other woman.
People, you included, should realize that a marriage isn't a bed of roses. It will have obstacles, it will have friction and unhappiness. The thing is how you overcome it.
It has always been this way since marriage was "invented". The curious thing is that people these days often choose to just embark in the next "great" thing. Often to find themselves terribly disappointed.