Do you agree that, other things being equal, a relationship in which neither partner would abandon the other in such a situation is probably a better one overall? What sort of qualities would make a relationship have that property? Are they more or less likely in a purely transactional relationship?

I agree such a relationship is likely better (although not everyone may want such). The most important qualities for such a relationship seems to me to be depth of commitment, and a sense of duty in each partner (to take those commitments seriously). They seem to me to be much more likely in a transactional relationship, where each party commits in return for the other party doing so too, than in a non-transactional relationship, where each party commits by an independent decision, whether or not the other party also commits.

If you truly cared about your partner "as an end in herself" you still wouldn't leave. Care to bite that bullet?

I'm not sure exactly what position you're arguing with and why you think it's my position, but: if my wife (I do, as it happens, have a wife) were unfaithful or became addicted to drugs, I would not necessarily want to end our marriage on that account. I would much prefer to salvage if it possible.

I'm not saying you'd necessarily want to end your marriage on that account. I'm just saying that you might (depending on how you feel about drugs, whether it was salvageable in a manner you considered acceptable, etc). Is there really nothing she could do that would make you say "I've had enough"? Because if you truly cared about her as "an end in itself" then it wouldn't matter what she did. Indeed, even if she ended her relationship with you and took up with someone else, you'd be equally keen to make her happy. Which, frankly, I don't believe. At the very least, if it's true for you, you're an exceptional person. The transactional analysis says that you try to make her happy in exchange for her making you happy. Which is why when one person quits the relationship, the other person finds someone else to have a relationship with. Isn't it miraculous how people change what is their "end in itself" to precisely coincide with their mutual advantage like that!

Obligation, on the other hand, doesn't seem to have any place in a non-transactional relationship; as everyone was acting purely for their own ends to begin with, there can't be any debts or obligations.

does definitely seem to indicate a difference in meaning, though; I don't see how "non-transactional" implies "everyone was acting purely for their own ends" any more than "transactional" does.

In a transactional relationship, I promise to do X in exchange for your promise to do Y. So if I do X, and you don't do Y, you owe me. But in a non-transactional relationship as defined above, I don't do X in exchange for Y, I just do X because it makes you happy, which is my "end in itself." You don't owe me anything in return. Maybe you'll do Y because it makes me happy, which is your "end in itself." Maybe not.

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." It's not a good model of most relationships, and while it may apply to some relationships, those are clearly unhealthy.