Why does my fairly new boyfriend constantly joke about impregnating me?

We've only been dating a couple months and every time were intimate he later (or during) jokes about making a baby. He has a daughter (8) and I have 3 boys 5 and younger. We have a 10 year age difference. He said before he might want another baby but not once he reaches 40. (4 years from now). I don't know what to think of it. Obviously it's to soon to consider having a baby together... and he clearly wants me on birth control. So why so many jokes?
Why does my fairly new boyfriend constantly joke about impregnating me?
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