Sexualizing breastfeeding

I'm 16 weeks pregnant and my finance has been sexualizing breastfeeding in a way that I don't like. Anytime i mention that my boobs are growing or will grow or I bring up breastfeeding, he calls them "mommy milkers" and always talks about him trying my milk and making sexual jokes that honestly, I don't like. Im ok with him trying it, but I tried telling him I don't find it funny and to stop always thinking it's about him. The milk is for the baby and I just think he's being a bit immature. He says I'm being too serious and need to take a joke. I told him he needs to respect my opinion. Am I being too serious?