I'm a serial monogamist because I'm hoping to find the next relationship at some point. And that point in the future is very flexible in terms of time, and even in terms of what the person or the relationship looks and feels like.
If either of you decides not to do the work of keeping the love alive, then you're in for some tough times. And when negotiations and discussions break down, sometimes over sex, the fractures may end up becoming breaks. The loss of the love may end up signaling the loss of your marriage.