From Rosie Bell:
Your argument is very rational... but it's not how a novel works with a reader and possibly not even with its writer. J.A. herself said in one of her letters that her brother 'admires Henry Crawford as he should, that is, as a clever pleasant man'. Characters in novels take on a quasi-solidity out of the structure of their stories, and we become their friends and partisans and admirers. So, yes, though Henry C only exists in the world of Mansfield Park he has a quasi-existence in the real world and there we expect him to act in certain ways. In fact it is out of character for someone like Henry C to be attracted to a mouse like Fanny. Males deemed very attractive in their circle are attracted to females ditto.