If a couple is living with the understanding that the other person is a platonic partner, and both parties are very happy; it would be very unwise to introduced sex into the relationship or change the relationship from what it is. Your platonic partner is that for a reason. If it is working, it most often is not a good idea to try and change things unless you have given it very careful consideration and you have tested the waters to see how that person would react if you ever asked for them to be more than a platonic partner.
Both men and women benefit from this type of relationship. No one ever said that life was easy, and most people are fully aware that it is enormously helpful to have a member of the opposite sex as a partner. Even if no romance is involved, both parties will help each other greatly. A man may need the comfort and caring of a woman who enjoys to cook, will help him clean, knows who to iron, fix cuts he receives, trims his hair, can tell him what outfits match, give him advice regarding the opposite sex, can sew a broken button and many other aspects; the list is almost endless.
A woman can greatly benefit from having a platonic partner when she connects with a man who can help her fix her car, fix things in the home that have broken, help her move heavy objects, accompany her to events, install air conditions, check the oil in her car, provide a feeling of safety and protections and the list goes on; it too is almost endless.
Both parties in a platonic relationship will appreciate their platonic partner for being there to enjoy the many elements of a social life. They can watch movies together, cook together, go out to eat together, go for walks, go shopping, take vacations, go hiking and the list is only limited by their personal interests and hobbies.
There are times when a platonic partner can develop into a sexual partner; however this topic should be thou roughly discussed and both parties must be in agreement that they do have this desire and are willing to try this step up. A person should show very small and simple hints to their platonic partners that they wish to take things up a notch. If their partners responds in a positive way, they can then discuss their feelings. Some great relationships have started this way.