AIO over my BF saying he wants another girls kids?

22F and 24M, my boyfriend was texting his friend after we left a restaurant. He turned his phone around and showed me the message his friend sent, it read “I’d let — have my kids.”

He responded with the “😋” emoji, and said he showed me to say that the girl is fictional (in a show or something?) and that he’s open about his joke and doesn’t mean it, but I should look at her to understand.

Why show me if it was so harmless? It’s kinda weird to me, am I overreacting? This was a few weeks ago and it’s been stuck in my head everytime he watches the show with the girl in it (it’s an anime). I feel bad because he’s kind, and has never made jokes like that before. We’ve been together a year, but it’s weird to make jokes like that even over fictional characters.

He kinda compared it to me reading fan fiction which I guess..? But I wouldn’t say that about any characters in books I read