Secondary Affairs aka Fallback Relationships

I've always been inclined to categorize relationships people share,like the one here Desperate Relationships. Relationships would arguably one of the most complex forms of social interaction as a LOT goes into getting them started and even more to keep them going. Many a times people who are into relationships don't on the surface know why they got into it, why they are still in it and if they are getting what they had in mind before getting in. Relationships here include Friendships too.

Most of the categorizes I put relationships into are a result of my own imagination so don't be surprised if Google doesn't offer more on these categories :)
I've been observing quite a few relationships around me for some time and have reached a conclusion that having "secondary affairs or fallback/backup relationships" has caught up lately. I doubt if things were this way sometime back.To get some context lets consider a few examples:

1) A girl doing her Post Grads and in a serious relationship(plans to get married) with a guy for a couple of years is great friends with another guy and shares things with him that she doesn't/cannot share with her boyfriend. Not only this on the physical plane they share mutual attraction towards each other(just short of the first step) but they aren't in a regular touch and the frequency of their contact depends on how her relationship is going with her boyfriend. Her Boyfriend doesn't have a clue about this.

2) A Casanova in his early twenties despite having numerous flings hasn't let go of a girl who loves him. He sets aside some time and space for her on a regular basis. This has been going on for some years now and should unless someone decides to take a decision.

3) A married women who can share things with another friend that she can't even think of sharing with her Husband. Every time she has an emotional low she runs to him(think Emotional whore).

4) Heard on FM: A married lady with a kid has got into relationship with a guy because he stimulates her intellectually more than her husband. She still loves her husband and gives some time(and other things off course) to this relationship.

These relationships have attributes of what can be called 'extra marital affairs', 'two timing' etc but the main point being that people are in not necessarily for their carnal desires and they don't want to let go of the secondary affairs because they know in some senses these relationships are a surprise ingredient in their primary relationships and thus the success of one is kinda dependent on the presence/success of other.

Some attributes of a secondary affair could be
1) Emotional/Intellectual/Physical backup
2) The subject chooses to keep the details to him/herself and his/her partner is for most parts unaware about the comfort/chemistry he/she shares in this other relationship.
3) The time and effort that goes into these relationships is quite less as to some extent both parties know how it fits into their scheme of things and probably that's why they look so attractive.

What people fail to realize in the "cushion effect" of secondary relationships is that they are instead of working on the problem and making effort in a lasting solution are opting for an easy way out and a solution that can be temporary at best and might cause more harm than good. Having said that I am afraid but I see more of these affairs/relationships in times to come.