Dark Nights
Some nights, it's just Snape and Sirius.
Those are the darkest nights, when the children sleep and the adults either lock themselves in their bedrooms or go out in different missions. Nights where everyone assumes Sirius and Severus can remain in the same room without hurting each other. Like that's ever going to happen.
Those two like to hurt each other, a lot. Is during those long, dark nights that they push aside all pretenses and just let their most primal instincts come alive in a tangled web of sweaty skin and ragged clothes.
In the dark, they somehow bond.