You are wrong. That is not what religion is at all. How dare you. Religion is a good enough reason because that is where morals come from.
I know a lot of atheistic people with better moral than most "christians" I know.
BTW, comments like:
Are a little bit off here.
We dare.
We are in a religion fo0rum because we like talking about things other people don´t dare to talk.
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Topic at hand:
I think this question would depend a lot in the cases. Morality in relationship has a lot to do with intentions and attention to each member involved and it´s incredibly mutable.
What is immoral for one couple to do, would be immoral for another couple not to do.
I am sure that open relationships can be lead responsably and lovingly to each parti involved. I am also sure it can be carried disastrously and scarringly to everyone around.
It not only depends on if they are "adults" (read, above 21 in biological age) but a heckuvalot more to do if they are EMOTIONALY mature enough to handle it.
BUT
This is true with any marriage.
This one does indeed probably need more maturity in both parts for it to work, and also of course that both parts are happy with this.
children would grow up thinking that commitment and loyalty is unnecessary. That could adversely affect their relationships in the future...and if they have children of their own, their lack of commitment to the other parent may cause them to abandon their own children.
It is also of great importance to notice that you can love someone without having sex with him/Her and that trust and love are not based solely in with whom you sleep with.
Adultery is something that personally always revolved my stomach because to think that someone is capable of hurting a loved one´s trust only for some ephimerous gain makes me sick, it makes me ad and disappointed.
Now, if both parts consensually agree to an open relationship, there is no "trust" to be broken on that regard. They´ll just trust each other to be good life partners, parents, friends and advisors to each others for the rest of their lifes.
Of course everyone knows that last part has nothing to do with loyalty or love in any way close to what sex has to do, right?
Love is bigger than sex. If you only want the cow because of the milk then guess what? you like milk, not cows