The way I read your post, it seemed to be saying that there were specific acts of foreplay that were forbidden if lustful but acceptable if not motivated by lust, and that this distinction applied specifically to these particular acts of foreplay rather than to sexual activity in general. That was what was confusing me. But it seems I may have been reading more into your post that was actually there. If so, then I apologise.
If you are merely saying that the Catholic position prohibits both
(a) married people treating each other as objects, and
(b) some forms of sexual activity that protestants and/or unbelievers may find acceptable,
then there is nothing for me to be confused about.
That’s pretty much all I meant, sorry for the confusion.
BTW the protestant understanding of what sexual activity is acceptable between married couples that I’ve heard most often doesn’t refer to that verse. It’s more along the lines of "Tbe Bible is silent about what exactly married people may and may not do in bed, so we rely on a general understanding of the Bible’s view of marriage, which tells us that it must only involve the two of you (and thus no porn because that’s pictures of other people) and it must be loving (so only do things that both of you are happy about, and don’t try to pressurise your partner into agreeing to something they don’t want to do) and it must respect the believer’s body as the temple of the Holy Spirit (thus don’t do anything that injures or harms each other). For many, this third consideration would prohibit anal sex because it gets bacteria where it’s not good for them to go, and carries a risk of fissures etc.
I agree that the vast majority of practicing Protestants would not approve of “swinging” or “threesomes” in a marriage, but I have seen some defend using porn to “spice up” a marriage. That’s probably still a minority view, though.
Most of the examples I can think of Protestants using the “marriage bed is undefiled” idea as essentially a proof-text, is to justify some sexual practices that seem scandalous, but are not technically prohibited by a specific Bible verse, such as the idea of sex in a public place because “the Bible says the marriage bed is undefiled”!
I also see among some more conservative Protestants the idea that well, actually, the way God designed men versus women, is that men have a NEED for sex that they HAVE to get met, otherwise they will suffer, whereas women, not so much. And so wives are sinning if they don’t meet their husbands’ every single request both for sex in general, and certain sexual practices, even if they find them unappealing, otherwise they are guilty of the sin of “withholding sex”.
Some seem to really think that “God made men to lust, and see women as sexual objects, and there’s nothing wrong with that, as long as he only uses his wife to meet his sexual needs, that is the only outlet God allows”. The corollary to this is “if a married man cheats then his wife MUST have driven him to that sin by withholding sex”. (I even recall some people used this line of thinking to blame Josh Duggar’s wife for his adultery.)
ETA: A less outrageous example I have seen justified by the Hebrews verse, is whether it is okay to fantasize one’s spouse is, say, a pirate or a prince or some other “romantic” figure (and I guess a husband might fantasize a wife is a Hollywood actress, princess, French maid, etc.) and many say that’s fine as long as in the fantasy, the sex takes place in the context of marriage, and there is no “third person” involved because the pirate or prince or whomever is not a real person.