It's taboo because mothers are not supposed to also be sexual beings. You're either a Madonna or a Whore and not a multidimensional human being. Anyone who crosses the line between those two as a woman or as her lover, is considered a bit perverted. As you've noted, women's pleausure itself is somewhat suspect because it challenges the patriarchal expectation that women exist for the pleausure and enjoyment of men. Obviously, there are places where that's not outlook is not in place but it often is in the subtext because it's so deeply ingrained.
I'm so glad you wrote this piece and thanks for the conversation. It's sparked a story idea for me.