Why Wont my boyfriend have sex with me?

My boyfriend and i have been dating for 4 months now, we live together, sleep in the same bed, we do everything together including gym, school, and work! its not like we stopped having sex, we just have never had sex, so what im asking is has this happened to anyone else? and why do you think this is?

please serious answers only,

oh yeah i forgot i ask him why and he always gets really frustrated...

help=]