There are (human) apes that will. I think that you're underestimating how -for lack of a better word-, deontologically/morally, some people see things. Also (or maybe this is what I mean by deontologically) how much of a self image someone can tied up in being a person who doesn't break promises or how -for lack of a better word (that I can think of) literally people can think of breaking specific commitments.

Well, except the grinning. In any case if the marriage goes badly Kate is free to leave so unless it goes so badly she wants to string him along to torment him he can always ask her to end it.

also this, "I just don't find it credible that time-binding apes, faced with the seconds of their life ticking away in misery, will just grin and put up with it without massive outside pressure." They do.