"does anyone want to adopt this kid?"

Today I was at a barbecue with some people I don't know. A baby started crying and her father jokingly asked, "hey do you wanna adopt my baby? She's free!" That killed me inside. As a birth mom it hurts my heart when people say things like that. There are some days I would do anything to deal with my baby's tantrums. Maybe I'm being oversensitive. Usually when it's someone I know I gently educate, but if it's a stranger I try and just brush it off. Is there a polite way I can say that that's not funny or is it better to leave it alone?