r/K

"Other than possible lower birth rates (already an issue for economic reasons), what might be the root cause of such rabid hatred of intergenerational attractions?"

Intergenerational relationships are generally associated with higher birthrates, not lower birthrates.

Look up the Western European Marriage Pattern some time.

Of course, the lower birthrates also have some advantages, and the higher birthrates some disadvantages. Basic r/K selection theory, a question of quantity versus quality. If you've read Tolkien, think orcs versus elves.

Of course, intergenerational attraction could work with either reproduction strategy - it just depends on how it is handled.