There's a good thread about this in AIBU, here.
It's not so much the affairs per se (as lacking says feminists can have affairs - that's about being a "dick"), but more about the fact that he had those affairs with women he had power over (shades of the casting couch), treated some of his female actors really shittily, and, when one looks carefully at his writing, turns out to be not so much feminist as "fun-fem-for-ulterior-motives" (prostitution as a woman's highest calling, more than a touch of the madonna/whore complex in his writing, rape as a plot device...)