Mostly depends on how they became ex-anything, really. If you ended on a neutral or friendly note, it's not out of the question to things go like this or to rekindle the flame if you're still compatible. But if you ended on a bad note...
Awkwardness comes mostly because most people think exes are off-limits because you broke up for a reason allegedly still there and neither wants to go through that again. Sex makes it a bit more complicated, like you kinda crossed a threshold neither should have.
Still hopefully this story shows some maturity in these situations. Tentative follow for now.