They'll start with "They are consenting adults, so why do you care ?" as they did with homosexuality.
Not so long ago, it was totally normal to think homosexuality is disgusting. And nobody would blame you for saying it loud.
Nowadays, this is possibly the most offensive thing you could possibly say, on about the same level as saying the word "nigger".
It is very much possible that incest will be normalized too in a few decades, including the mother/son version, which would be insane if we consider that even bonobos don't break this taboo.