I don't care about the incredibly gaping flaws/plot-holes of this manga (most pretty much brought up in the first chapter alone...) when pertaining to this topic. Heck, I've made a topic for this series because it's pretty much the only example that I know of for what it decides to do. Thus, I'm just curious about this general question:

Why the Hell would you force your protagonist to have only one option he can choose from (in this case, being able to marry no one else but his childhood friend), so that he can live a happy life and not screw everyone else (in the world) over?

[Basically: Why would any author limit their protagonist's choice of free-will, to which any other possible option for him/her to be happy would result in his/her + everyone else's damnation, and run with it like it's a perfectly fine idea?]