Platonic Relationships

Are platonic relationships a good thing?
Are they possible?
How difficult are they to maintain?

Is it OK for a man, John, in an exclusive romantic relationship with a woman, Jane, to have a non-sexual relationship with another lady, Ann?

We consider that following situation:

Ideally, John’s relationship to Jane involves friendship, romance and sex.
Ideally John’s relationship to Ann involves friendship but no sex.

I think it would be very normal for Jane to be at least somewhat jealous of Ann.
No one is totally secure. Jane would wonder what is the true relationship between John and Ann. Ann may actually have, or develop, sexual feelings for John; after all she likes him as a friend. Also John may actual have or develop sexual feelings for Ann; after all He likes her as a friend. And couples have their ups and downs: John and Jane may go thru times when the “magic” is gone from their relationship. After over-exposure to Jane, John may find that he is sexually attracted to Ann.
If a sexual attraction develops between John and Ann, should they break off the platonic relationship? Or should they continue on the idea that attraction is normal but they can be strong and not yield to it, that their relationship is too important to be destroyed by acting on sexual feelings? What if Ann is very sexually attractive --- is it reasonable for Jane to think there is no sexual attraction between John and Ann?
Is it believable that John is not attracted to a sexy Ann? Is it believable that, if John were attracted to Ann, he would be not act out?
Does John have the time and energy to enjoy romantic relationship with Jane and the platonic relationship with Ann? Or does he find that he has no energy left for Ann? Would John have a tendency to compare Jane and Ann?
When John starts up the relationship with Ann, is he being honest to clam that it is platonic? Or is that a cover for his real intentions with Ann? Does John know, himself, his real intentions with Ann? Can Jane trust that John is only interested in a platonic relationship with Ann? Can Ann trust that John is only interested in a platonic relationship with her?
What kinds of things can John and Ann do together? Can they watch TV alone at Ann’s house? Can John every have Ann over to the house with Jane? Does John have Ann over to the house when Jane is out? What is John’s motivation? Why does he want or need a platonic relationship? When does John tell Jane about Ann? Is Ann someone that John met on his own or is she a friend of Jane’s?
How is John’s time divided between Jane and Ann? How does John manage his responsibility to Jane and Ann?
I think that most people believe that platonic relations are a normal part of life. But on the other hand we are all aware that they are many challenges with them.