here's your one friendship right here! My very best friend in the entire world was once my lover, something like 20 years ago, and we are still best friends to this day. It's totally not a problem for either of us, or for our spouses. Not impossible at all. You know it for sure, do you? HA! My friend and I are soul mates, but in a strictly platonic fashion. There is totally no reason for either of us to ever be separated (except by miles), and no reason for us to ever become or want to become lovers again.
I said it is practicly impossible, which means there is a small possibility. I have a best friend. We hang out together every day for at least 3 hours.. Even more on weekends and in summer for the last 20 years. And our friendship is still strictly platonic. Another example of something not so often. But how many ex couples that are now friends (and only friends) do you know?
Im still friends with my ex wife, in fact we get on better now as friends than we did as husband and wife. Weird but without the stress of our marriage we are actually good mates. She's also become one of my fiancees best friends. So yes its certainly possible.
I am at the age where I say no it really isn't possible, but maybe in a few years my thoughts will change.