There is always a valid reason for sex, if you need sex.
@5128gap
Your differences are still inequalities.
Sex without equality inevitably ends in one party feeling used, FWB is a term created to make sex unacountable.
I've seen affairs covered up in the guise of FWB, from the 51 year old man shagging his 28 year old secretary, to the yummy mummy whose recently divorced using some others woman's husband to get her mojo back and get some new wardrobes fitted as an extra benefit.
All fine cause the're FWB.
To the young girls who are fobbed off by some young hunk who's a babby daddy, whose got no intention of being with his numerous FWB's.
Or the cultures in which it's fine to shag someone but never be in a relationship or marry them as the family wouldn't like it.
There are a million reasons why couples cannot be together, usually it's because of unequality.
Unequality very rarely makes both parties happy.
If someone said to me I want a FWB with you, I would run a mile, equally if I said that to a man I'd hope he would too.
It's basically meaningless sex, I would also state that if a friend wanted meaningless sex, they are not really a friend.
Do people actually stand face to face and tell each other "you are my FWB" I don't think they actually do, they tend to call them that when describing the relationship.
And they do that because face to face it would be quite insulting.
By the way meaningless sex is fine but call a spade a spade. FWB is a term coined to make the unequal partner feel slightly better about being used.