What is marriage?
'I believe marriage isn't between a man and a woman,
but between love and love'
Frank Ocean's comment in the pic at the head of this thread is so open ended as to be meaningless. You can use the term 'marriage' in all sorts of contexts - it is often used in company mergers, for instance, and 'love' in any meaningful sense is conspicuously absent - but in the sense of a permanent relationship between two human beings it is much more than just 'love' (which, by the way, in the above quote is undefined and can mean just about anything).
In the sense of a permanent relationship between two human beings marriage means so much more:
It is meant to provide a lasting and comforting companionship that lasts a lifetime.
It is meant to provide a place for a mutually satisfying sexual relationship.
It is meant to be a safe place where both can develop themselves into the best persons they can possibly be.
It is meant to provide for the procreation of children. Though not all married couples are blessed with children, most are. The survival of the human race depends on this!
It is meant to provide a place for children to be reared in a place of safety in a context of love and respect.
It is meant to provide a picture of the relationship between Jesus Christ and his church.
It is quite obvious that many marriages do not fulfill many of these things, but that doesn't change the purpose of marriage.
Only a lifelong relationship between a man and a woman can fulfil all of these purposes. Only a man and a woman together can bring children into the world. Children need both a mum and a dad to find proper relationships in the world. Mums and dads both love their children, but they do so quite differently. Children have need of both types of love. I could go on ...
That is why marriage can only be between a man and a woman.