if you truly cared about her as "an end in itself" then it wouldn't matter what she did.

This simply isn't true. I can value X "as an end in itself" and still give up X, if I value other things as well and the situation changes so that I can get more of the other things I value. Something being intrinsically motivating doesn't mean it's the only motivating thing.

This non-transactional model of relationships implies that it's a mere coincidence that couples happen to have each others' happiness as their arational "end in itself."

If you mean logically implies, this also simply isn't true.

It might instead, for example, be a result of being in a relationship... perhaps once I become part of a couple (for whatever reasons), my value system alters so that I value my partner's happiness as an "arational "end in itself." " It might instead be a cause of being in a relationship... I only engage in a relationship with someone after I come to value their happiness in this way. There might be a noncoincidental common cause whereby I both form relationships with, and to come to value in this way, the same people.

More generally... I tend to agree with your conclusion that most real-world relationships are transactional in the sense you mean here, but I think you're being very sloppy with your arguments for it.

You may want to take a breath and rethink how much of what you're saying you actually believe, and how much you're simply saying in order to win an argument.