Why are the laws of niddah called the laws of tohoras mishpacha/family purity? Certainly the "tohorah" part of the phrase makes sense, but why isn't it called "tohoras ishus", marriage purity, or something along those lines?
I actually had a discussion about this with someone this morning!
As noted here, this term is a very recent innovation. The conclusion of the person with whom I was speaking (a recognized Talmid Chochom who's name I'll leave out to preserve my own anonymity) was that it is a term that western sensitivities are more comfortable with. "Family Purity" sounds much friendlier and more balanced than "Purity for Women." As for Family vs. Marital, I actually hear both tossed around, although admittedly Family is much more common. I don't think there was such a science to the adoption of the term.