The point of it was to show where same sex marriage sits in a higher level of judicial review due to it being based on gender, not sexuality, then group marriage or multiple marriages would.
1st Tier scrutiny includes race, religion, national origin
Strict Scrutiny: The government must show that the challenged classification serves a compelling state interest and that the classification is necessary to serve that interest.
2nd Tier scrutiny includes gender, illegetimacy
Intermediate or Middle Scrutiny: The government must show that the challenged classification serves an important state interest and that the classification is at least substantially related to serving that interest.
3rd Tier scrutiny includes everything else, including sexuality (although this is believed wrong by many, it is where it currently is) and number of people in a contract or number of contracts that a person may be in
Rational Basis or Minimum Scrutiny: The govenment need only show that the challenged classification is rationally related to serving a legitimate state interest.
So when arguing from the gender basis on SSM, it would not be comparable to group/multiple marriages because it is at a different level of scrutiny. To make the argument that group/multiple marriages are comparable to SSM would be opening it up to make the argument that interracial marriages are also comparable to same sex marriages, because they are all on different levels of scrutiny.