Like any relationship between a mentor and child, it's not equalizing.
I think the idea behind sexualizing/romanticizing Cass' fascination/admiration for Bruce is that it takes her out of the mentor/child relationship. It doesn't add to the relationship, it changes its nature. Suddenly Bruce isn't a god-like, perfect figure anymore; he's a man Cass idealizes because she's in love with him.
I don't think it's either fair to their relationship nor very interesting, but that's how I interpreted the quote when I read it. It was much less ambiguous than the scene above.
it makes even less sense to change the story to one of romantic attraction between a father figure and daughter
To be fair, the attraction was only implied on Cass' part. Unless you wish to read Batman's overprotectiveness of Cass regarding Superboy that way, but that's another can of worms.
Also, I found the source of the quote I gave earlier, and it's from Scott Beatty's run on Gotham Knights, so definitely not Horrocks' fault.