Some quick background: we're in our early thirties (30M/30F) and have been in a relationship (not married yet) for several years. I'm a bit more go-with-the-flow, and partner prefers clear-cut plans with generous communication. Although it doesn't come naturally to me, I make an earnest effort to be very mindful about following through on my commitments with him the way they were originally planned, and communicating early on about any potential changes, etc. Was more of an issue early on in our relationship, but hasn't come up very much in recent history. On the flipside, he's grown a little less rigid, which has helped.
Yesterday, we agreed to take our dog on a short walk in the evening. We customarily take him out to play every afternoon together, but thought he could benefit from a rest day after observing some stiffness. After dinner, I was feeling a little more tired than expected, and we wrapped up a little later than I had envisioned (so it was fully dark outside at that point). I wasn't too eager to take the dogs out when it was so dark. I know that my partner doesn't take well to last minute changes in plans, but figured it might be OK in this one instance since we've been so in sync on plans in recent history. He's also been a bit more receptive to slight changes, so I was optimistic. I asked my BF whether it would be OK if we actually skipped the walk today. He agreed, but was visibly unhappy. I respected his disappointment, and apologized for the last-minute change. I explained that I hadn't thought the timing of our evening very well, but did not want take the dogs out when it was so dark.
I didn't really expect for this to be a big deal. But the sulking never stopped. He ended up doing work for the next 4-5 hours, and not really talking to me. I tried to give him space at first, then asked if I could do anything to make him feel better, then asked about work, etc., but was given a pretty cold reaction. At that point, I didn't feel like this was an instance where I should overly coddle and beg for forgiveness, so I didn't. We ended up going to bed without much conversation.
This AM, he told me he thought it through and had been extremely disappointed by my changing plans, and would really appreciate: (1) an apology, since he "didn't recall receiving one"; and (2) more follow-through moving forward with plans with the dogs. I apologized (again, even though I had done so the night before), and we coordinated to take the dogs out today. But frankly, I'm disturbed by this whole interaction. Was what I did so heinous? Is he reacting from a past trigger, rather than this specific instance? His reaction seems a little disproportionate to me, but am I being insensitive for thinking that? I can't put my finger on why, but I feel like the way this went down was not very healthy. I feel like I am entitled to initiating a change in plans every now and again, even though it's not expected, without being treated with such reproach afterwards.
TL;DR: Respectfully requested that we change plans to walk the dog, because I was tired and it was a little later than I had envisioned (my own fault, perhaps). Boyfriend reacted with extreme disappoinment, and treating my coldly for hours. In the AM, requested that I apologize and show more initiative/enthusiasm with plans moving forward. I am unsettled.