Why did he seem to be upset?

I was telling my boyfriend I am considering changing careers and going into trades as there are a lot of programs to help women get into it, he even shared a few links to help me. I then jokingly said "will you still think I'm sexy if I do it?" and he kinda got upset and told me he's not answering me anymore and to stop being so hard on myself to which I told him I'm sorry and that I was joking and he said okay. Why did he seem to get upset? I was only joking around cause you know plumbing is not a sexy job at times lol

Why did he seem to be upset?
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