How should I deal with sexual comments ?

F20 I was Chilling today with my two classmates a girl and a guy . And me and the girl were scrolling through clothes online. And we saw a push-up bra and I was like oh let me see this. The guy just said you don't need it ,it's not like you have any. And I didn't see that one coming cz we ain't even that close I just sat speechless and gave him a look. I just didn't know what to say cz I did fear looking insecure in a way or another. But i hate getting such comments from guys , friends or not. I joke with a guy for 5m and suddenly they see me like a "bro" they make hurtful comments concerning my physic. Is there a way to respond without looking insecure and assert boundaries at the same time ?. I hate being in this position. I just don't know what to say