Thanks for the responses
I am not disturbed that he laid out the limits for G. I don't think I asked him to, but I think that it makes some sense to let someone know these limits rather than a vague "can't do that day/time/activity" that may just leave new partners confused or feeling jerked around. I'm sure he did so in a way that was respectful of me. I'm sure he made it clear that these are *our* agreements, not my rules.
The point of contention was that he mentioned to me that he told her, but really didn't tell me the full truth. He could have not mentioned it at all -- I would have been fine with that.
We've talked more about this. He agreed to try thinking a bit more before speaking. He doesn't think that he consciously left that out when originally telling me, which is possible. I agreed that I will try to back off when I'm feeling insecure and not launch into interrogation mode, which obviously makes him feel defensive. (That wasn't the precursor to what happened this time, but a more general problem.)
There were some questions about the situation more generally. My partner (S) and I are in a long term committed relationship, with things that often go with that: shared finances, shared home, chores, meals, pets, annoying (extended) family events, telling each other when one of us has bad breath, etc.
S is in an extremely demanding and stressful profession, meaning that we often struggle with both equitable sharing of household responsibilities and having a lot of quality time together. (I also work rather long hours, but S is expected to work most evenings and weekends.) The limits on S's time with G are about preserving our time together, his ability to get work done, and his ability to contribute meaningfully to household tasks. As they stand, I think they're pretty generous. However, he's only been seeing G for a week, so it's all very crushy and he'd likely spend a lot more time right now without the limits.
Over the course of our relationship, we've had a very small number of external sexual relationships. Neither of us is looking for another serious commitment, but S wants something that lasts longer and has more substance than unattached sex.
My insecurities about this stem somewhat from the fact that S and G work together. The fact that S can tell me "I'm going to the office" and that means he'll likely see her throughout the day, maybe take a break or two for coffee, maybe have lunch together makes me feel very threatened. I think this is about the fact that although I could say "no" to a specific date night, I can't interfere with his work. Previously, he spent a lot more time working at home and we get those sorts of moments together. Now, he goes to the office more often because he wants to see her, so I feel like I'm losing a lot of moments that were very enjoyable.
There are a few other things that make this hard to swallow: the fact that he's making time for her, but is responding to some to my requests with "I'm super behind at work." I also realize that this crushy stuff will wear off quickly, but have trouble getting that to information to change how I feel in the moment.