This isn't love. They both know that. The ones they loved are gone, and all they have left now is the fight, and each other.
Harry doesn't care. He recognises that everyone needs to take their comfort where they find it these days, and he didn't blink an eye when he found out about them.
It doesn't happen every night, just most nights. Most days, news of more deaths comes in, and they seek each other out to bury their sorrow in passion.
This isn't love. Cho knows that. But when she's lying in Sirius' arms, it feels enough like it to numb the pain for a while.