Platonic relationships: How would this work?

Question

So I've been think about this, and came up with a dilemma:

Persons A and B have dicided to have a platonic relationship. Everything is going swimmingly until person B tells person A that they want to have/have got involved in a just sex/FWB with someone else as well.
Is this cheating or does it not apply since sex wasn't part of the original relationship? What woul you do as person A?

Answer

It's up to the couple to decide what they consider cheating. Personally if I was in a sexless (by choice) relationship and they wanted to seek sex elsewhere I would be okay so long as it doesn't go beyond physical.