They were already eligible to marry.
It changes the definition of what marriage has been in America.
So?
Why shouldn't two brothers be allowed to marry and take advantage of the benefits? Two consenting adults, hurting no one.
Sounds like you have some parades to organize. Best of luck.
Changing the definition opens the door. For example:
Please explain why two brothers, two consenting adults, shouldn't be allowed to marry. To take advantage of the benefits. How will that harm you?
Because the definition of marriage isn't being 'changed,' no 'door' is being 'opened.'
Marriage remains a commitment of two equal, unrelated, consenting adult partners recognized by the state – same- or opposite-sex, it makes no difference. Same-sex couples are currently eligible to enter into marriage contracts; marriage unchanged, unaltered, and not 'redefined.'
That's not the case with two brothers wishing to marry, where no marriage law is written to accommodate siblings.
Consequently, your 'argument' fails as a slippery slope fallacy, it's nothing more than unfounded, inane demagoguery.