Is this a recent development? Masturbation can involve more friction than intercourse, so maybe this extra stimulation is what's doing it for you. In that case, abstaining for a while like Vance2335 says might help. Or if you're taking certain medications, there are some that can delay ejaculation. Another explanation I've heard of is a decrease in sensitivity if you're circumcised - but that's more a gradual thing that men notice over the years. There's also the mental aspect to it - if you're sufficiently "in the mood," it may get you there faster.
How long do you have intercourse for?
Have her stop using her hand to finish you and you don't masterbate either. Wait 1 week for sex and see what happens.