I am with D insofar as I don´t actually believe into vanilla people - I think everybody has hidden fantasies one way or another.
My first real relationship broke because we discovered our likings at the same time - guess what, we were both submissive and into pain, but neither of us wanted to "hurt" or dominate the other, it just felt horribly wrong. Looking for things outside the relationship was not an option - we both did not function like that (we were 19 y.o., so no long marriage or whatever was at stake, but the breakup was painful as hell nevertheless).

What I read in some other threads coming from submissive ladies who complained like hell about their vanilla hubbies learning the Dom stuff at such an agonizingly slow pace or refusing completely - I found that very heartless. Your husbands married you vanilla. How would you feel if halfway through a marriage your hubby came to you and said "become something you never were, something you might not even like to be, or else I will cheat on you or leave you, or in the best case demonstrate at every given opportunity that you do not make me happy?"
Sometimes reading those threads I have the feeling that the vanilla spouse´s feeling never get considered.
That sucks. Really. I have no way of putting this nicely.

And apart from that: I can assure whomever that if the man does not like to be a Dom, it will never ever truly satisfy the submissive - it will always feel stale and weak and not genuine, compared with the real thing.

I was lucky enough not to consider anyone vanilla as a spouse ever after that first catastrophic breakup in my life. My heart is with the ones who realized what they are only too late, and no, I don´t have a solution to their problem... What I do know is that, unlike some people might believe, BDSM is not simply a "kink" one can do without... it goes far deeper, and not getting it can make one seriously unhappy.

I would think it fair if such a half-vanilla couple would end the relationship in a non-cruel way, as this way each of the partners (!) has a chance to go and find someone that makes you truly happy, but that´s a choice one has to make for oneself.