“Ages of experience have taught humanity that the commitment of a husband and wife to love and to serve one another promotes the welfare of children and the stability of society.”
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But whether a couple is a man and a woman has everything to do with the meaning of marriage.
Marriage cannot be severed from its cultural, religious and natural roots without weakening the good influence of society.
Marriage is an institution fits in perfect harmony with the laws of nature whereas systems of slavery and segregation were designed to brutally oppress people and thereby violated the laws of nature.
If we change the definition of marriage to be more inclusive, then it is logical to argue that we should broaden the definition so that won't exclude anyone.