Really I have not found any better example of healthy and...dare I say pure S&M play anywhere for any kind of relationship. Gay, straight, younger, older, this series just outlines exactly why this isn't just some weird fetish people force upon themselves.
Nana to Kaoru is another example of a great S&M manga. Though Kaoru is so ugly he makes sin look good, the relationship between the two main characters is extremely sweet. They can't communicate by normal means, so they do so with S&M. It is dialogue and thought heavy though, lots of reading with that manga.