Their Most Closely Guarded Secret (смешанная)

It started out innocently enough, their little arrangement. A delicate balance of comfort and propriety. As close as three of them were before, no one blinked when a fourth seamlessly joined their group. No one thought to look closely enough to notice that perhaps they were all a little closer than they should be. A little more intertwined than was considered healthy
Or: the slow evolution of a taboo relationship between two couples and it's eventual dissolution.