"Do me a favour."
With this he pushes Peter into an empty classroom, a broom closet or – rarely – onto his bed. Peter likes him and can somehow understand him so doesn't think about it. Peter wants to be a good friend, so he doesn't talk about it, either. And, really, it's okay that way. Peter's life is simple.
"Friends do each other favours."
He yanks his pants up and leaves. Peter waits a minute or two and casually strolls away in the opposite direction, as if nothing had ever happened. And so he can still look Sirius in the eye.