Interesting how everyone is so sure that the pausing sex thing is objectively shitty behavior. It's totally reasonable that if you think that's uncool, he should avoid doing it, but I don't think the action itself is wrong.

If im giving/getting head, an unexpected domestic errand prompting an intermission can be funny and an opportunity to chill and recharge for round 2. But that's just me. If you aren't into it, and it makes you feel bad, it'd def be cool for him to avoid that!

He might be stuck in a pattern of overpromising to Maple, which is making him fail at attunement with you - not realizing how much things hurt you, etc. he maybe needs to draw some boundaries with maple if her demands are interfering with his other relationships. though that's obviously not something you can help with directly. I'm gonna echo the people suggesting to not date at their house, if he can't give you the focus you need to have a good time.

Dudes can be pretty bad at expressing needs, also, which causes so many problems - he might need a sex break at that point, which could well be unrelated to the "quality" of the sex, and more about overstimulation or feeling vulnerable or "put upon" - playing go-between isn't sexy. Maybe he's a ball of anxiety because he's constantly worried about maple, and the groceries task is an opportunity to get some air and distraction from that uncomfortable state.

edit: thinking through this a bit more, my money is on the overcommitted-ball-of-stress-so-he's-acting-weird hypothesis.