Sooner or later you will be at a crossroads. Either of the following can happen. Or, if neither of the two gives in to the sexual attraction of an outsider, and doesn't even admit it, boredom and indifference will set in. The one sure thing is, it won't last on and on. Even if the relationship will be continued for decades, because this is dictated by the conventions of a place, or because other options just don't seem feasible, it will, after a while, no longer be based on love.
Procreation is a biological necessity.
We all are wired to fall in love during our youth. To fall in love raises the appetite to copulate, and to copulate is necessary in order to procreate. There is also a biological necessity for feelings of love and care. Where there is only appetite to copulate, offspring may not be attended to sufficiently. In my view, nature is terribly efficient.