My uncle Billy was once asked whether he preferred little boys or little girls He replied "*Well, it would be little girls, because you can turn them around and now you have a little boy*"
Why is your uncle telling you this, and why are you telling me this?
I don't know, I think you have to decide
While I see that you and your uncle may have your own dark humor going on between you two, I do not think that others would see it fascinating or funny and they may see it as exploitative specially if the joke is trying to have sex with little girls or little boys. I suggest that you urge your uncle to have jokes that are more appropriate, or try to have a more wholesome stories to tell that all your family members could enjoy.