Catherine wouldn't turn around the next minute and reject him (a true Gothic heroine would no doubt have done so, that is- after fainting away at first...;-). If Henry had told Catherine about the rift with his father before proposing, she would have been placed in a very awkward position. She probably would not have held out too long, but it would not have been an unequivocal and joyful acceptance- da man did the right thing!

JA uses a similar device in Emma when Mr. Knightley proposes: for as to any of that heroism of sentiment which might have prompted her to entreat him to transfer his affection from herself to Harriet, as infinitely the most worthy of the two -- or even the more simple sublimity of resolving to refuse him at once and for ever, without vouchsafing any motive, because he could not marry them both, Emma had it not. She felt for Harriet, with pain and with contrition; but no flight of generosity run mad, opposing all that could be probable or reasonable, entered her brain. Emma Ch. 49