Ok first of all I have been on both sides of the coin. Being the one not wanting sex and being with partners who didnt want sex. So I aint "Taking sides" and know how much it hurts being rejected and also know how annoying it is being pestered.
From my experience I seem to have learnt a few things. One of the first things is that if your partner has gone from wanting sex a lot to not really interested, usually there are a few main factors.
1) Unhappiness with the relationship.
For whatever reason they are building resentments towards you. Maybe they feel you are not pulling your weight or that things are not 50/50. Before you flame me and tell me you work your butt off and massage her back every night I just want to add that her resentments towards you are subjective. Just because she has them it doesnt mean she is right. In other words, maybe she is taking you for granted and not noticing all the effort you make. maybe you are not pulling your weight. Only you can know this. However this reason is usually one of the main reasons why women (or men) go off sex with their partner. So, the only way to solve this is communicate of course, find out what her needs and wants are, decide if she is being reasonable in her requests, and either follow through continually (not just until you get laid) the other option is to conclude her requests are unreasonable and not follow through. Things will stay the same, or a split will happen. Now please rest assured I am not saying do whatever it takes to please her. Thats crap and should not happen. Just as many women are selfish or unreasonable or greedy as men and she could well be being unreasonable. Thats up to you to decide.
2) Tiredness
I know right! You have heard this one a lot. I am not defending it, once again just pointing out that this is a major reason for sex dwindling. However I can give you a womans perspective on what we mean by tiredness because it is not that simple really. Its a mental tiredness, and in a lot of cases is more to do with just completely being in the wrong frame of mind. Something men are really good at, and us ladies need to learn more from you on this, is that men will make more time to chill out and relax. Its a good thing. The X-Box, Golf, Lads night out, Hobbies. Men can reach a point in their day or week where they can go "fuck it, its my time" As long as it isn't a stupidly large amount of the week it is a good thing. Woman dont ever seem to switch off. I am talking from experience, I am one :D. You know 3 or 4 thoughts are constantly spinning through my head at any one time. Like, are the kids school uniforms all dry ready for tomorrow, shit I forgot milk at the shop, what day does that bill go out, I think little one is coming down with flu, I best get a doc appointment, what do I need from the shop for dinner tonight".....on and on it goes. Yes we are good organisers, the food is magically in the fridge and clean clothes hung up and all the other things that maybe you take for granted as a man (Not all men, just generalising again) The point being...How on earth do you get into "The Mood" when sex has almost become the last thing on your mind and when you have 24 hours to sow all the badges on little ones cubs uniform in time... You just dont feel sexy, baring in mind that you all know women are thinkers, and need to think themselves into "The Mood" and need more time to get into "The mood" So when a woman says she is tired. I am willing to bet it isnt all physical, and that the tiredness she has is half mental, as well as just not being in the right frame of mind at all. Yeh, it sucks, but unless we find a way to make women plan, organise, worry and think less then I guess this will always be the case. The best way I can think of comparing it is like, for example your hobby is fixing cars, you have a car on the jack and working away and for the life of you trying to work out what is wrong with it, your brain is coming up with all these possibilities and you are getting frustrated and your missus walks in and says hey can you give me a hand job please..... hehe (Waits for you all to say "I would" haha!
There are other factors but those two are the main culprits I think. However I do object to the thinking that woman use sex to trap men. I mean, thinking along that logic, guys often go out their way to wne and dine and spoil and treat a woman at first. Then that stops. Does that mean men use money or treats to trap a woman? I think you should act as you mean to go on and although we all make extra efforts at the start of relationships, it is unfair to try and impress a woman by splashing the cash and later, when things go sour, accuse women of just being interested in money. Just the same way it is unfair of women to accuse their guys of being perverts or worse when they continue to want sex. Not going to lie we enjoy being treated, but so do men and it should be 50/50 in this day and age. And remember it like this, you are comparing the sex you get now to the beginning of your relationship. Do you still treat her exactly the same as at the start? The flowers, the love letters, the mixed tapes or dedicating songs to her, the surprises (And no they dont have to cost money) the sweet words and constant touches, the running over at 1 in the morning and staying awake with her because she got scared etc etc......or do you now tut, and fart and continue to play the x-box if she asks for a cuppa? :P Women need to be in the right frame of mind with you. Sorry but it really is true. I guess in the bedroom department that makes us harder work. We are sorry! :P
I do not think it is healthy to have a relationship without sex and obviously there are issues that I can see from your posts you have tried everything. If you really have tried everything then I guess you got unlucky with a really selfish person and that sucks. It should be 50/50 but have you also thought about your place in the relationship. I mean, OLD...you say two partners lost interest so where does that leave you, firstly you assumed they trapped you and then you gave three options for yourself. One was to cheat, the second was to cheat and pay for it and the third was do it yourself.....
Is there not another option, to talk and compromise and find out why it has gone so bad and try to work together for a solution. Are not the options you gave just as selfish as what you say your partner is being? Or have you just reached a complete stalemate, where any conversation about it just doesnt get resolved or turns into accusations or arguements and you are so tired of trying? Because there is then another option, couples therapy. Which is less expensive than an affair :P I mean you say no decent man would walk out on his responsibilities and commitments, but if you are thinking of cheating, thats worse than leaving. surely?