Caleb sound like a fine person. Though imperfect--as it is with most everyone--, it sounds like the two of you have good communication with one another.
Your posts have me thinking about my own exploration of polyamory. I've not yet had a lasting addition to my "romantic" life, just a sad ending to a short-lived love affair with a man who was previously my friend and who ultimately even ended our friendship. (Now we are basically on small-talk terms, since we are next door neighbors in a duplex appartment.)
Anyway, my mainsqueeze of about a dozen years and I have for some time embraced polyamory -- in theory. Neither of us has had a lasting and significant 2nd partner. So the two of us haven't had a lot of opportunity to be challenged by the other falling in love with another--and having that be mutual and sustained. The challenge that will likey result should one of us have such a sustained "2nd" is the movement from "theory" to "practice" concerning nonpossessiveness and non-fearfulness that this 2nd indicates some sort of depreciation or rejection of the other. In theory, we're good with this. In practice it seems likely that he or I will on occasion feel some rejection or non-appreciation--even though we theoreticlally know better. By which I mean that we're both on the same page as to the question of multiple loves: we understand that having two loves does not halve the amount of love! (The explorations we've had with others--brief 2nds--and discussed with one another actually had the effect of opening us to *more* love between us!)
I'm saying this as a sort of riff on what you said about your Sundays with Caleb, and the dishonesty toward you that followed--but which was apparently discussed and resolved. He was probably afraid that you may feel some rejection or abandonment..., if he was honest and direct with you about his choice to spend time with his friend that Sunday. That same fear may have been doubled by the fact that the two of you usually spend Sundays together. I think he was wrong to be dishonest and legitimately hurtful to choose to spend so much of Sunday with his new love-interest. But not wrong, or less loving, to open to a possible new love. And that's the trick, for me, for many of us, to keep these issues segregated. Our society profoundly inclucates us with messages which equate "romantic" love with monogamy. It wouldn't be surprizing to me if it took any of us years of effort to see through the fog to the clearing where the truth lies.