sexual disfunction is a known side effect of SSRI's. your doctor should have warned you about them before he/she prescribed them, but it wouldn't surprise me if they "forgot" to mention it in hopes you would take your pills.
i took prozac for a while and did not suffer any problems. if anything, feeling better made me more horny than ever. as farbio says, once your body evens out a bit, your problem may go away. i'm not sure exactly why increases in seratonin levels mess with your sexual functions, but from what i understand, people on E (basically a super-SSRI mixed with meth) also don't perform well sexually, despite feeling all touchy-touchy. might be an interesting thing to research.