Could somebody explain to me how someone who doesn't consider it a moral issue to have sex before marriage faces some huge moral dillema when he wants to "cheat" with someone he's not in the sacred dating relationship with? When did dating become bound by a sacred covenant as if it's a marriage?
I think most don't have moral dilemma of which you speak. I think it isn't that hard to understand really.
If in a more 'sacred type dating relationship' it becomes less simple to understand.
Issue with monogamy as (I would say dogmatic) practice is it can, rather easily, become all about 'specialness.' Or what Gollum would call 'my precious.' It is not really about trust or commitment, but on the surface I understand for many it is 'all it is about.' It is more about one or both partners putting this relationship above all others (all others) and when one or both do this, then cheating is seen as violation of the special relationship. Attempting to destroy the magic that is created in that special bond.
The 'sacred covenant' (even in dating) comes from how ego uses relationships to get what it needs. There is more to say about this, but I'll just leave it with idea that ego needs to fool (its) host into thinking this relationship is best version of 'oneness' that a human can ever hope to achieve.