Why am I still reading this thread? LOL ... since I stated my opinion and pretty much accepted that a couple of the posters here simply had a different opinion, and that was that.
I have to say, I pretty much agree with Hetta on a lot of points.
There was a time when, due to illness, my husband wasn't supplying what I needed in marriage. There was no real companionship, no intimacy, and no sex.
At that time, I had a family member (kind of) helping out. We had been friends for many years, he was actually a double-in-law (no blood relation) and I thought of him more-or-less as kind of a brother. We had gone out a few times (not as a date or anything), shooting pool, that kind of thing. Just relaxed and had fun together. I've done this with my brother as well.
At one point in time, I can't recall the reason exactly, we were both tired, and ended up lying down on a king-size bed. Both fully clothed, on top of the covers, each on our own side, not touching. Just resting, and we both slept. When I woke up, I recognized the intimacy of the situation, and to me it had crossed the line. In a way I WAS tempted, because it had been some time since I got what I needed in my marriage, and I had "loved" this man (in a familial, platonic way) for many years.
This was a danger to my marriage, and I immediately put distance between us, no longer finding companionship with him and certainly not sharing the proximity of a bed. We did nothing inappropriate, but it could have led to such (in my mind) and I felt the wise thing was to recognize that and stop it immediately.
I hadn't recognized the danger at first because he was "platonic" to me (as I said, practically a brother in my mind) and seemed safe.
Maybe those kinds of things wouldn't happen to another person ... but I don't think that's likely to be true. I don't think I'm unusual in that. I think MOST people would begin to be tempted in a situation like that.
Why? If you acknowledge that you can not get all of your needs meet by one person why must this specific need be supplied by ONLY your spouse? If your relationship is good otherwise and your SO does not feel threatened by this interaction why is it an issue? Why is it presumed that you should only have this level of intimacy with one person?
What I'm imagining from your scenario is a marriage where - what? They live together, pay the bills together, have a household in common. But the need for intimacy is met outside of marriage by another person? That's not marriage in my book.
Of course I am acknowledging that that is MY position. And I think one that is shared by many. But you, or anyone else, always have the freedom to do what you think is right.
If the question is one of Christian morals, I don't think this falls within what is right in a Christian marriage. Outside of Christ, the world will always do what it wants to do, and that is not my place to legislate, but between every person and God.
(ETA: I realize my last sentence may be taken in the wrong way. I am NOT saying that everyone outside of Christ would do this. Not at all. I realize most non-Christians have morals that would make this wrong in their books as well. I mean no disrespect. My point was simply that I was posting from a Christian perspective, and as such, I don't believe it is our place to morally enforce the world outside of Christianity, especially on strictly interpersonal matters.)