Do you think this is a sin in itself, or because of where it came from? If she made up the phrase herself, is it then permissible?
"Where it came from" is the primary problem. Because "where" or "what" a particular device "came from" is what is being brought into the marriage
, when it is employed within the marriage. If a man watched porn or had a pattern of fornication prior to marriage, and requests that his wife look or act in ways that appease the preferences he learned from those activities... he is bringing his prior porn use and fornication into the marriage
(EDIT: or even if the preference was established simply from having his lust aroused by seeing/hearing women dress/behave/speak in immodest and overtly-sexualized ways in public without ultimately obtaining gratification
from them... there is still that whole "lusting after in his heart" thing - it's still an issue. He is bringing prior lust into the marriage.)
That being said...
I think that the things married couples will naturally invent on their own between themselves in a healthy and God-ordered context, will always and necessarily appear and sound QUITE DIFFERENT from the aesthetics popularized by Satan's minions for the commodification and degradation of sex.
The former will be the accentuation and appreciation of what is already there; of what they have naturally
The latter is the imposition of an artificial aesthetic outside of the natural order.
In the former context, a woman might fashion and make for herself a very simple gown that only bares her shoulders and knees -- and the allure of it will simply be that it fits her form very precisely. It accentuates her body
In the latter context, a woman might go out and buy an outfit that fits her so-so or even poorly, but has a lot of pre-packaged allure because of specific design elements that the man is pre-habituated to associate with arousal based on past exposure to and indulgence in degenerate imagery and situations featuring those elements. Classically conditioned response.
In the former context, a woman might ask her husband to say something during lovemaking that is already consistent with his vernacular. Something that naturally builds on her feelings of love and warmth, conveys his desire/love for her, etc.
In the latter context, a woman might ask her husband to say a pre-packaged phrase that she has learned to associate with sex and arousal via exposure to degenerate media, or from prior "encounters" with other men that she found particularly exciting (common "dirty" talk, etc).
In the former context, a woman might ask her husband to do a particular thing during lovemaking that is an extension of or re-combination of their natural interactions that make her feel particularly good. This could be as simple as the warmth/pressure of touch applied to a typically non-sexual part of the body during lovemaking, which combines known "warm fuzzy" feelings with the enjoyment of marital union.
In the latter context, a woman might ask her husband to "pull my hair!" or some other such reference to popularized acts
meant to cause hyper-arousal by association with existing notions of "excitement" built on exposure to degeneracy. Or because some other man (who excited her more than her husband does) did this and established the association between that behavior and amplified arousal/excitement.