I'd like to see the scholarship behind that, if you suddenly remember where it came from and can put your hands on it easily (I'm not asking you to spend hours hunting for it). It's a major assertion that I'd need to see more justification for than just some-guy-I-read-once-said-this!
I have always looked at the 7th and 8th commandments as paralleling the 10th commandment. Don't take your neighbour's wife (7), or his stuff (8), - actually, don't even think about taking his wife or his stuff (10). When looked at with that parallelism it becomes a simple matter of theft.
However, that isn't the whole picture, because it leaves open the question of "what if you do X with his wife but don't steal her", or "what if he permits you to do X with his wife". It's far too limited. We know from scriptural examples that sex with another man's wife is sinful even if she is not taken away from him completely. So adultery is broader than this and does cover any sex with another man's wife - however we define sex.
There is also the parallel of idolatry, which also helps us to see that adultery is taking service that is due only to a woman's legitimate head, and giving it to someone who is not her head.
I don't see any way to shoehorn "humiliating or degrading" into that definition. It seems an entirely different matter. I can't find any suggestion of this in Brown-Driver-Briggs or Gesenius' lexicons either. So until I see some evidence of that I'm going to set aside the idea that it could refer to any "humiliating or degrading" sex, and stick with the concept that adultery is simply taking another man's wife in some way.
Incidentally, this is a great way to teach the concept to children! It lets you explain adultery even to kids who have no knowledge of sex, and without using the word "adultery" and confusing them. Too many kids are taught "husbands and wives should not commit adultery", which is simply a way of reading it to children while deliberately leaving them confused as to what it means because you don't want to talk about "icky" sex stuff with them, so use a word they don't know the meaning of. Far better to tell them "don't steal other men's wives", or even "don't break marriages" as that is an even clearer broad concept that they can understand. But I digress.
I like your approach of looking at the exceptions. My answers for them may be inadequate, but here are my thoughts:
Clearly the wife is not being stolen from the husband, so the first parallel is not helpful.
But if we parallel this with idolatry - would God be happy to allow us to offer worship to idols provided He could watch and enjoy seeing us dancing around and so forth? Obviously not.
So as marriage parallels our relationship with God, this has to be considered adultery.
Adultery, in every lexicon, is defined as sex with a married woman. In every scriptural example of adultery where the identity of the parties is given (I have listed them all before to check this), the woman is married or betrothed. And the above parallelisms also show adultery is with a married woman. Therefore I think it is entirely reasonable to say something is only "adultery" if the woman is married (or betrothed) to somebody else.
However, adultery is just one type of sexual sin, "fornication" being a catch-all term for more sins than this. Promiscuous sex with an unmarried woman is not "adultery" - but may be fornication. I'm not excusing it, just being precise with the terminology.
I would say that if someone has sex (ignoring the question of what "sex" is) with an available, unmarried woman, they have an obligation to take her as their wife. If they fail to do so, they are sinning. But the sex itself was not the sin - the sin is the failure to follow through on the obligation created by that sex. So I am not condoning that sort of behaviour in any way whatsoever.
The sex cannot be sin against a future spouse, because if they had followed through with their obligations there would not have been any future spouse that they were transgressing against - they themselves would be the spouse. So the sex is not committing adultery against a hypothetical future spouse, because at that point they themselves are the rightful future spouse and cannot be sinning against themselves by having sex with their own rightful future spouse! Yet abandoning that person is sinful - and having sex with the intent of abandoning the person afterwards is sinful for the same reason.
I promise to post here on Biblical Families where I learned about Exodus-era interpretation of adultery involving humiliation if you promise to post the scholarship behind your assertion that we are to interpret the commandment about adultery through the lens of idolatry.