I don't really know that that's true anyway. It doesn't seem to be a frequently discussed topic IME. There are related topics like "should divorce be available" and "should people be encouraged or discouraged from pre-marital sex" where I think they are extrapolating "if you support extra-marital sex then you also support single-family households and must therefore believe that they have no disadvantages" but I think that's a larger logical leap than I would personally make. It also doesn't seem to account for support for access to abortion which arguably counter-acts the support for experimentation with pre-marital sex...
The general "left leaning" mood is also to provide services that help single-family households to mitigate issues for children anyway, so I think it's much more of a nuanced "lesser of two evils" position to support things like divorce being available.