1. Is there a such thing as 'Sanctity of Marriage'?
1a. Why or how come?
Yes. Sanctity, or sacredness, is about being set apart. Marriage sets a relationship apart from other, non-marital relationships. Marriage is, by it’s set-apart nature, sacred by definition.
The Set-Apartness of Marriage
2. Does it apply to every marriage?
I don’t think so. In a marriage of legal convenience with no real relationship between the “spouses,” the actual relationship has no real set-apart-ness. It is only illusion.
OTOH, I haven’t thought much about this, so I might see it differently if I thought about it more.
> What about cultures that aren't your own?
I can’t speak to cultures that aren’t my own. I don’t know enough about them.
3. What types of things constitute 'Sanctity'?
I don’t know how to answer this. The daily life things that make a marriage real?
Others have replied that sanctity involves being set apart specifically for religious purposes. I don't believe this, primarily because most of the things typically meant by "religious purposes" are cultural and human-made, and thereby artificially limited and limiting.
The Set Apartness of marriage creates its own sanctity, beyond our small imaginations.
4. Are there different definitions of Sanctity of Marriage?
Almost certainly. For example: Blessed by God. Inviolable. Exclusive. Trusting bond. Commitment.
5. Should your definition of the Sanctity of Marriage be forced on all marriages?
Forced? No.
But this idea of Set Apartness is sort of already hanging out there in the legal code and in cultural recognition.
6. Are open marriages, e.g. swingers, possible in your view?
Yes.
6a. If not does it mean that swingers can't love their partners fully?
n/a
6b. Is it still cheating even if both spouses agree?
No, not if they are keeping agreed upon boundaries AND continuing to take care of the relationship between the spouses.
7. If your definition requires religion is it possible for non-religious marriages to have Sanctity?
My definition does not require religion. Yes, it is possible for non-religious marriages to have Set-Apartness.