If you didn't see it in that genre, it's because usually, japanese authors just imply it. OF COURSE, the father is going to reject their child's homosexuality. It goes without saying.
Japanese authors usually go for the subtext. They write it in a way that the readers have to make their own assumptions, which are usually correct. It's a very japanese way of writing. What's not exposed is still implied and the author counts on the reader to understand it.
In Lily Love, Ratana Satis exposes it and tells it plainly, without subtext or any call to the reader's thinking.
That's what is so "shocking", because some readers here, who are used to manga, are under the impression of being told the obvious.
I guess it's a different way of writing, maybe because she's not japanese.