If you were diagnosed with pre term labor your dr should have put you on pevic rest, no sex or orgasims. Orgasims cause your uterus to contract which can set you into labor and semen can dilate your cervix. Bottom line like Ashdub said, don't have sex.
I agree with AshDub... I read that you shouldn't have sex if your at risk for preterm labor. Somethin abt tha orgasm or semen can trigger labor!