I don't necessarily agree with the other posters on this one. I do agree that a quick "no" would be off-putting -- why not take a minute to talk about it? -- and there are other nuances to consider in terms of who was in the right, but I'm going to go ahead play devil's advocate here.
I've read accounts on this board from newer partners who've felt like they had no safety or standing in their relationship because, among other things, their metamour would cancel their dates with their partner because of "emergencies" that could have been solved other ways (and indeed, he found another way to handle things, by having her help out with the medicine). In light of this, to keep from sabotaging your partner's other relationship(s), it seems like one should try never to ask to cancel dates at the last minute except for true emergencies.
So the question for me is, was this really an emergency where you needed his help right away and no other solution was acceptable? Or was it just a matter of your preference that he be home because you were stressed and it would have made things easier? If the latter, it would have been reasonable for him to cancel, say, plans to do some hobbyist activity, or even a planned night out with friends. But a date night with a partner has emotional significance to both parties that shouldn't be easily shunted aside.
Why, btw, are we calling it "sex time" and making this about his dick, rather than calling it a date and making it about their feelings, whether or not the intention was for the evening to consist partially, or even wholly, of sex? That seems sort of weirdly disrespectful. Am I misunderstanding the nature of the connection -- are there no emotions there?
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.