for me it is not excuse. i think married couple should not get bored sleeping with the same person and have sex with him/her forever.they should make they night hot but i know some couple separate about it.
hmmmm.....maybe they just fall out of love or they are not just really meant for each other that's why!
no, it seems you will do your best, but you have no idea what he will do it's not because you think about it like this, he will too (and keep on doing this after marriage) he may not care anymore after a while, either he meets someone else, or life changes (new job, new responsibilities, ..).. it's possible after a few/many years that you do your best and see it has no effect at all anymore, you also give up from then you start to live separate lives then breaking up may be the easiest for both
its good to keep on changing your taste .. as you are born once on this planet and its good to share every night with a new partner