The difference is that (1) it's officially unsupported to use *any* Apple-written library that is not part of POSIX in a program which also uses fork, (2) since Apple considers this NOTABUG, the APIs provide no way to work around the problems.
With X11, using fork is fine -- you just have to make sure that you only use that connection in one of the children. The bug report I linked to is Apple explaining that you cannot use their *linear algebra library* (which implements the standard, cross-platform BLAS interfaces) on both sides of a fork, and that the only solution is to use a different linear algebra library.
[NB I don't use OS X or really care about it either way, except that these things are annoying to work around.]