They're supposed to be a couple here, in the "aww, aren't they sweet together" sense. While that is sometimes split apart from yaoi and put into boys' love or shounan-ai, my understanding is that at Danbooru they should be tagged as yaoi rating:s.
Specific visual cues: riding together, their expression, the lace in the background and yes the way Lelouch is holding onto Suzaku.
Also, the image is tagged as "suzululu" which seems to be pairing tag for the two of them, and the artist has made several other lelouch and suzaku as a couple images. I know these two reasons don't fit the "tag what you see, not what you know" ideal, but I think they can be a helpful deciding factor when unsure if an image should be tagged a certain way or not.
