'Marriage' is the institution
Who wants to live in an institution?
in which sex between a man and a woman not closely related should be confined. Nothing else can be called by the same term.
But it already is. You're too late - you're behind the times - the world has moved on for the better and history has passed you by.
The question then becomes, why are your beliefs so at variance with fact? (Though that would take us far beyond the present discussion, for unravelling why beliefs are so at variance with fact would take us to an explanation of religious belief itself).
Your "should be" is no doubt an expression of your own typically narrow, cramped, illiberal and ugly beliefs by which you personally are perfectly entitled to live, but thankfully are not binding upon anyone else.
It's an ideal based on facts not taste.
Which facts would these be? Given that you've just condemned yourself out of your own mouth that your beliefs do not in any way, shape or form rest on facts, so you're not exactly well placed to spout about facts.
Your "ideal" may well be yours, but your unfounded opinions - again - do not dictate the life choices of anyone else.
I am
tremendously happy about that fact - and in this case, it is one.