Santa,
As we say in Spain, "Each one talks about mass(religion) just how they get in with it"...more or less literal translation.
My experience has been wonderful so far. I really don't know what would be without the legal and religious bindings, but I do state that the "ties" have not been a problem at all.
I agree with you parents (both sides) are a problem, especially the first years. I'm a lucky guy, I have both mine and hers 600 Kms. away, so our run-in period has passed just fine, no interferences.
As for why is the marriage is so great, well...I focus it like a house. By far the interior is the most important and, at the very end, it's the part you can't live without. They are the feelings, the moral compromise and, above all, love. The legal, religious, social ties are the exterior. The part of the house seen by others. My personal take in this is (coincidence?) the same as for my real house. I like it to be clean, simple and beauty. Beauty is on the eye of the beholder, and so my marriage is beautiful for me. I don't care about what others might thing about it.
You can live a life with or without marriage. As you can live in a house with an imposing facade, a large garden, or in a tent, in the middle of a wood. Both perfect. Both with his pros and cons.
It's all a matter of personal beliefs and, thus, personal options. Each one has his own personal and unique arrangement that fits. The only tricky part is findin it. I've been immensely lucky so far.
Cheers,