I'm going to post this....not to scare you, but to let you know there is a possibility of sex causing labor. This was my experience.
On new years eve 2011, I was 30 week pregnant. DH and I got it on (unprotected) in the morning. ABout 30 minutes later I was feeling VERY srong cramps that started to increase in pain and speed. I went to the hospital and I was in labor. They were able to stop it and I was put on bed rest and given nefedipine to stop contractions.
Doc banned sex, but said 'manual stimulation relations' were okay. In fact, I had lots of relations this way with no issues! Fast forward to 37 weeks, the day I was released form bedrest and DH and I got busy (unprotected). 15 minutes after, I was in labor. DD was born 18 hours later!
Thinking back, I am sure it was DH semen making contact with my cervix that caused me to go into labor. Semen has prostaglandins that are very similar to the ones they use for induction.
So this time, DH and I are using condoms throughout the pregnancy to try and prevent preterm labor from happening again!