Sirius knew he wasn't in love with Remus Lupin.
He knew that it wasn't love because he felt wonderful. He wasn't miserable, he never regretted anything he did with Remus and he never spent his time second-guessing every move that either of them made.
He knew it wasn't love because his heart didn't stop beating whenever Remus did something. It beat as strongly as ever. And he didn't spend all his time daydreaming and wandering around in a daze. Yes, he was happier when Remus was around, but he didn't keep thinking about him when he wasn't.
It obviously wasn't love because every moment he spent with Remus he spent on Earth, not on Cloud Nine. He was always there; always aware of his surroundings, the person he was with. He knew it wasn't love because what he had with Remus was so sweet that it couldn't be love.
Sirius knew he wasn't in love with Remus Lupin.
But he did love lying together in a tangle of limbs with him, exhausted, satisfied, happy, and watching Remus smile right back.