Re: Explain a Fetish
The general perspective of 'Netorare' is of the guy (or girl) whose lover is being taken from them. Of course, like with all terms, it's sometimes just used interchangeably for sexual activity that relates to infidelity. I've come across a few works where the protagonist has to discover their mother or sister being... 'stolen' away from them and yet they've somehow earned the NTR tag, for whatever reason. In addition, it seems that the object of the protagonist's desire doesn't always HAVE to be in a steady relationship with them.
Then there's 'Netori', which is from the viewpoint of the one stealing one member of an established relationship away for a night or more. If the person being NTR'd does NOT find out about his significant other's infidelity at some point, I'd probably say that the series aims towards the viewpoint of the 'netori' (stealer) or even the outside observer. For the latter, one reason that NTR might have appeal is because 'spice' in a story, emotional context, makes it mean more, rather than just two bodies fucking. Drama at work can be arousing for some.
'Reverse Netorare' is when a heroine or other female character steals the male protagonist away from the girl he has an established relationship with.
I'm not sure about the term for the viewpoint of a girl who is being stolen away from her man, but that is also a fun one that tends to fall under the NTR umbrella.
The reasons for this fetish are fairly diverse and, as you might imagine, fairly controversial. If you care to look more in-depth into the matter, just do a search on articles about 'cuckoldry', as that is what NTR is, in essence.
The topic has fascinated quite a few evolutionary psychologists and biologists, and it isn't hard to find an academic paper on it.
(change the x's to t's, of course)
hxxp://www.epjournal.net/wp-content/uploads/EP05358362.pdf
However, as a something of a fan of all of those topics (Netorare, netori and reverse netorare), I'll try to explain it in my own words.
I guess you can call it something of an 'emotional / mental BDSM'. Some people get turned on by whips and hot candle wax, while that is a bit too kinky for others. Same deal with netorare, I would argue; learning to handle a degree of mental anguish is more often an acquired sort of fetish, but once you get used to it, it isn't too bad.
Some other fans of NTR argue that they simply like to watch the object of their affection receiving as much pleasure as possible, even if it isn't necessarily coming from themselves.
On the other hand, netorare is NOT strictly a beta male thing, contrary to the belief of many laymen. Some serious alphas from prominent fight camps are said to have allowed other young, virile males from their gym to sleep with their wives--so long as they can watch, of course. I feel this has a lot to do with competition, or a very competitive mindset. You want to see how other men might try to please your wife. Either way, she's likely having a good time, but in the process you can see what you might want to improve on or where potential inadequacies lie--or maybe where you do better than others in some areas. While some submissives are into NTR (or rather, cuckoldry in general), there are also some very dominant and competitive folk who enjoy it.
I've read several accounts of cuckolds who have never been so vigorous, or have cum so hard, than when they fuck their wives after her pussy is already sloppy from the cum of other men. Evolutionary psychologists might also theorize that there is some sort of sperm wars thing going on there, but I'll leave the details of that sort of speculation to them. There's quite a bit of reading that can also be done in that regard.
It takes a certain degree of emotional and mental resilience to be able to 'handle' netorare, I would argue, since obviously you don't want to be completely destroyed by having to watch a loved one do something like that in front of you. Of course, it isn't for everyone; some people who are better at handling jealousy still won't go for NTR, simply because they haven't acquired the association of it turning them on.
Just my two cents.