I've heard about this, but never saw it inside. This is where he says that an Eishes Ish is b'etzem assurah to her husband, too, but there's a hetter where he's legitimately fulfilling the purpose of the marriage. The teshuva also has another tremendous chiddush that would render useless all the gramma switches and Shabbos stoves, which, by the way, is what the Feinsteins hold.
It's not OCR'ed, so I can't copy it, and you'll have to read it inside. You wouldn't believe me anyway unless you saw it in situ. It's in the Warsaw edition of his Teshuvos, Teshuva 18.