I think that quite often the FWB thing is a phase that either leads to a relationship, or not. I haven't seen it continue on in any kind of satisfying way.
Even my husbands best friend who is very mono and very much a player has become fed up with dating many women at once or just having casual sex. Trouble is for him is that he doesn't know how to do anything else and is stuck.
Yes, eventually it is boring and pointless. Sex is better anyway when there is depth of relationship there.
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