"Plural marriages are a separate issue."
Same issue - redefining marriage to "not discriminate" against the "love" between two consenting adults.
"It was decided on the principle that the state could not impose certain ristrictions on consenting adults who belonged to a specific group."
That's only a part of the answer. The other half is that the state can impose restrictions if a distinction can be demonstrated. Additionally, race is not a valid trait for discrimination because the Constitution prevents it - there is no such protection for sexual behavior / preference.