MY BOYFRIEND WONT HAVE SEX WITH ME

WE HAVE BEENT OGETEHR FOR OVR A YEAR an he had been i iraq for a mo he comes hme we have sex for the first few wks an now the past 2 wks straight we have had not had sex...wen i want to he always makes an xcuse,an its always a adiff one an the thing is wen we r messing around he gets hard an as soon as we got to have sex he gos completely limp lik he shoiws no interest in sex with em at all only kissing an touching... not evn oral sex!!! his xcuse is he is nervous btu if he wud really be nervous wud we not have ahd this [problem int he beginning of our relationship or rite after he came bak from iraq!? wat shud i think!? is he cheating on me or is there really a logical reason!!!