How do I tell them it makes me feel uncomfortable?

sex skeeves me out and my boyfriend's friend and his girlfriend have no problem talking about it when my boyfriend and i are hanging out with them. we don't talk about our sex life with them, so why do they talk about their sex life with us? my boyfriend doesn't say anything about it, maybe just chimes in a couple of words in the conversation. i may be the only one who has a problem with this (even though i know my boyfriend's pretty conservative too). how do i tell them that they're making me feel uncomfortable the next time they start talking about sex and i don't want to hear it? how do i do it without making anyone feel bad?...especially me for being a "prude".