There are folks who feel anything but the traditional family unit is just plain wrong, and should be explicitly outlawed.
I’m not one of those people.
Thing is, after that, anything goes really. The only rules are “do no harm.” That’s a tall order. I’ve been in a situation where I was dating someone in one city, while rekindling a long-dead romance in another.
I should have been in hog heaven, but actually in a very short period of time I found the whole situation very distressing, even when I knew all parties were aware of the hanky panky and voiced no strong opposition. I guess, in the end, I was after a monogomous relationship, and indeed, that’s what I wound up with. Strangely enough, it was the less convenient option (the long-distance romance) I chose, since that’s where the deeper emotions lay.
I look at the polyamorous and think: bless 'em, it looks like a whole lotta fun. In practice, I couldn’t get my head around it, even when one of the parties was three hundred miles away. I think there’s something in human nature that tends to make us most comfortable with coupling. For those lucky few who can sustain a sasifying polyamorous life long term, I can only say, hats off to you; you have my deepest envy.
Me, I just can’t do it.
Comfort level, lee, is the likely culprit to explain your question. It’s just easier to deal with when the sex is out of sight (and hence out of mind). For some it might be a good middle-ground between our simultaneous need to make intimate bonds, and our sometimes overmastering desire to fuck as many people as our horny little selves can manage to. Sex with the same person for… the …rest…of…your…freakin’…LIFE…it’s a sobering prospect indeed. But having my wife and my lover in the same house all the time…eyahhh! I’d lose my mind. But a spouse who says “hey, go have your fun if you must, but I don’t wanna see it, I don’t wanna know it, I don’t wanna even smell it,” is something many Europeans have been doing for ages. It’s how they keep their marriages together, it seems. Generally, it’s a triangle (or a pair of triangles) consisting of your spouse and best friend, plus a fun little fuck partner on the side who, if you are extremely lucky, you don’t fall in love with.