I'm not so sure she did.
Sounded like an odd thing to say, so I tried to track it down. It appears what she may have said is that "the marriage bed is the most degenerative influence in the social order." Which is quite different, or may be, in context.
So I then tried to find that phrase in her book, Women and the New Race, that was cited as the source.
Not in there, at least not in that form.
She may have said this, or something quite similar, of course. If anybody has the actual quote in context, I'd be interested in seeing it.