I think you should feel deep regret for what you did. I know I would. For what it's worth, with deep shame, I can also say I was the "other man" once when I was 16. As an immature teenager who'd never had much attention from girls (catholic school, all boys), I became involved with an friend of a friend (of a friends) girlfriend. Everything blew up, and I didn't speak to her for many years. Then, wouldn't you believe it, we finally got together properly later. We had an 8-year relationship, bought property together, and then, wouldn't you know what happened? She ran off with my best friend.
You could say I got my karma. I could have relied on excuses like my age or naivety, but I don't. I acted like a piece of shit and got my just desserts. I've learned from this and would never do anything like that ever again. I personally believe consequences and karma helped my development. What consequences and karma have you experienced? I do think it is morally distinct from cheating, at least to my view, as cheating is betraying someone you love (have loved). Being the other person is about having empathy for people you don't really care about. It's a much more abstract emotional intelligence to develop. To my mind, cheating seems a far worse act that being the other person for those reasons. Though you could argue that this opinion is both controversial and biased. To expand on that, it's for the same reason I would judge someone far more harshly who stole from their friends or family than that of a stranger. It's clear to me though that both acts are highly immoral.
It's natural if you have a sense of morality or are working on developing one. The scenario you've described is one of the most heartbreaking and immoral acts one can participate in while still being legal. Every detail was like a blow to my view of humanity as I read it. The disabled child, the STD, the threesome, the age gap. Quite frankly its enough to drive the betrayed spouse to suicide. I hope very much that she's still alive and well. I can hand on heart tell you I'm not aware of anyone in my life who has done anything half as bad as what you've described. Though you read many as bad or worse here. So of course you have regret. I too have regrets.
I would steer you away from considering the married person in this scenario, though. You can only control your own actions. Don't worry about their remorse or lack thereof. Perhaps he's a sociopath and incapable of feeling remorse; that's not really your concern. In my case, I suspect the other party was a run-of-the-mill "monkey brancher"—when things got tough or boring, they moved on to the next person, not risking being single by keeping their partner around before making the leap.
The following is a psycho-analysis made from some basic observations. I'm in no way speaking with authority on the matter. Please disregard should you be uninterested in my inadequately informed opinions.
My advice to you would be to examine your relationship with your father (or perhaps the lack thereof) in therapy. I've got to say, you are exhibiting what sounds like a run-of-the-mill father complex. From the age-gap relationship you are currently in, to degrading yourself in threesomes with older married men, it seems very clear to me. You are so desperate to fill that father-shaped hole in your life that you are using unhealthy habits to bridge that gap. I'm always reluctant to suggest that "waywards" focus on self-esteem because it seems like such a cop-out. However, in your case, you need to be aware that you are worth far more than a figure who'll spread their legs. You are trying to replicate father-figure relationships and you feel the only tool you have to engage them is to sexually entice them.
Putting aside the father complex, the self-esteem issue also crops up in your post: Struggling to fix the "whys"
You yearned for adult companionship and because you think so little of yourself, you used sex as means to accrue it. I suppose this would all be subconscious. I'm aware you won't be actively thinking: 'why would he want to be friends with me? best offer him an orifice to keep him around' but I'm argue this subconscious self perception could have been a contributing factor to your affair. Once the sex begins, all the typical affair chemicals kick in and perpetuate the thing
With regards to fix this, no short cuts I'm afraid. Personally I don't think you are a safe partner yet but you can work on being one. Review this in therapy. Furthermore, I concur that female friends should be your target in the near future until this issue is resolved. Just for safety. Based off people with similar personality profiles as you, Is it fair to assume you don't have many female friends?
This is just food for thought.
[This message edited by DRSOOLERS at 10:29 AM, Thursday, September 11th]