If someone tells you that they want a "friends with benefits" arrangement they are telling you; "I love having sex with you but (you) are not (the one)!"
Clearly a "friends with benefit" arrangement is a major "demotion" in relationship status when compared to being a spouse.
Note: Not all desires for change are the result of a "midlife crisis". Sometimes people experience a "midlife awakening" where they reach the conclusion that if they keep doing what they have always done their life will never improve. Don't fall for the old;
"I love you (but) I'm not (in) love you" line.
It's simply another way of saying:
"I care about you but (you are NOT the one)!"
"Never love anyone who treats you like you're ordinary."
- Oscar Wilde