You can also be 100% effaced for weeks before anything happens...
It's not the dialating that's the most important. It's effacement or thinning of your cervix. You can be dialated to 2-3 for weeks before you deliver. If you're not thinning then nothing is going to happen. Sex helps if your near your due date. It's the prostaglandins in the semen that softens the cervix. For me sex isn't very comfortable or enjoyable at that point, but it does help.