Quite a complex issue this one! I've been thinking about 'all things love' for many many years, and I'm trying to boil it down now to just one point in this comment.

I guess that one point would be this: I worry people take advantage of each other, while pretending to have good intentions. Feminism, for example, talks about the many ways in which men degrade women in their minds. From the woman's perspective, all may be well. But if she could see inside the man's head, all would *not* be well! 

As a man, I grew up with men, and know how they talk about women they slept with behind their back, and how they think of them. If I was in a relationship with a woman, I'd worry she's not able - not "smart" enough? lacking the experience or someone who grew up with men, obviously? - to tell the good guy from the bad. You talk here of 'special connections' and such, but let's be honest: in most cases the more attractive people (the ones who tend to be assholes precisely because everything was handed to them on a silver platter) will get the most booty. 

I don't now how to express this exactly. I know most men will just be having sex for the experience, like "I want to know what a redhead fucks like, what a Hispanic girl is like in bed, how a black girls likes to take it" etc.

This sounds extreme, but I'm just trying to illustrate my main worry: people take advantage of each other. When a person is with another for a long time, they will naturally start caring for each other like they do for themselves. When it's something casual, there's no investment, it's easier to put on a nice face and just sleep with someone. Psychopaths are pretty good at this. I'm sure lots of women would find Hitler very attractive and eloquent. 

Most men, when a woman approaches them who's with someone, will naturally be guarded. If she tells them "oh it's fine, my boyfriend doesn't mind", they will naturally think to themselves something along the lines of "I can't believe my fucking luck" and "I can't believe how stupid she/he/they are". 

I'm fine with people doing it who give the same as they receive. But I worry most people will only be benefiting while putting on a nice face. And I don't think most people really accept what goes on in these relationships. One example is the one you gave about couples who 'prefer not to know'. Why?! It's clear they're not really okay with that. I often wonder why for example polygamists don't just get together and live out their dream world. Well, it was tried, by the flower children, and failed. I think if you put polygamists in the same place they'll eventually just slaughter each other. I worry most of these kinds of things are just people lying to themselves, and to others. But I also worry the same is true for monogamy!

This feels like a rant!