Quiet types were harder to get to know. They held things in. Remus did, and Sirius had to try hard to figure it all out. Getting him to talk was the start of it. Then saying something that was a sort of a guess helped because it made Remus react, and that told him more. It would have been a lot easier if Moony simply told him. But Sirius Black knew full well how rarely things came the easy way in life.
"And maybe you'll decide you'd rather not have me," Sirius shrugged. He didn't think it particularly likely - hadn't spent any time thinking about it. And if it happened, Sirius would deal with it then. In the meantime, Remus was his. "I'm glad you know," Sirius kissed Remus's skin, "And when your silly fears come, you can just hop into my bed." Cuddles, shagging, whatever.