Fundamentally, they are unstable, like three legs on a table. Something always goes wrong, or at least it should, because triangles usually end up hurting people more than anything else. Triangles exist simply because a problem is not being resolved in a marriage. Two people get married and have marital problems, instead of resolving those problems either by fixing the relationship or ending it, which by the way can be a valid resolution of a problem, a third party is brought into the relationship, and now you have a triangle.
But I think there are plenty of people who never should have gotten together to begin with. A lot of misery is avoided with a respectful ending and opportunity to start again with someone else after a period of sincere efforts to fix a love relationship.
Instead, defensive stuff happens like distancing from each other or finding another lover to make up for what is not happening in the marriage. Too many pieces for stability what you get instead is fragmentation, conflict, and limited intimacy. The target of desire in the relationship is the woman.
The marriage is active and bonded. For a time this may feel freeing and easy. He may start thinking about asking her to leave her husband. This is when the trouble starts happening. My heart goes out to him more than any one else in this arrangement because he is usually the one who gets disturbed first and has more to lose.
He could be thrown out of the triangle and the married couple could end up fixing their relationship, this could happen. All three people in this triangular arrangement have their issues.
It only brings this issue roaring to the surface. In my mind, this is an opportunity for growth because triangles are not good for people. The work needed is usually in the area of developing a better tolerance for love and intimacy by clearing out whatever fears and blockage the guy has in the way. Then he can go after a whole and available single woman.
Anyway, the married man like his supposed competition is afraid of a deeper intimacy as well. They have this in common. His willingness to live in a limited marital relationship thinly disguises this fear of deeper love and intimacy. The question is whether or not the married man will fix this intimacy issue with this wife. In this kind of triangle with two men and a woman, there are instances when the two men fight over the woman. This kind of drama is amusing only because if they consciously knew how connected they both are and how much they have in common they might realize that it would be more profitable for both of them to have a conversation together about the whole thing in a coffee shop or something.
That way they would learn about their love life issues and probably resolve the triangle with less pain, skipping the whole painful machismo thing.
Another interesting dynamic I find in this kind of triangle is, the married man could be functioning unconsciously in a pseudo-fathering role for himself and his wife. The clues for this kind of dynamic can sometimes be found in their sexual relationship. If sex is kind of weak and the romance is minimal, that could make it easier to create a father-daughter bond out of a husband-wife relationship.
OK, on to the married woman. I left the best for last because in this particular triangle, with one woman and two men, the triangle is really under her control. Her psychological issues are running the show, or more precisely, the triangle.
One is stressful enough these days. This is the kind of situation that ages people. If she could recover from such an evening and say, this is crazy! Unfortunately, most triangulated married women opt for a resurgence of effort and energy into getting better at controlling the double lives they are trying to lead.
As a psychologist, I know that double living is toxic. Unfortunately, a lower percentage of married women in triangles have that insightful need for change I just described before the triangle falls apart against her will.
The common ways in which this kind of triangle falls apart are: The married woman has to either recommit herself to her marriage or grieve the loss of it. This will take emotional priority over starting a new committed love relationship. People need to leave something emotionally before they can recommit emotionally to something else.
Otherwise the past, unfinished emotional business is always threatening to invade the present.