Yes, its fine. Your cervix closes to keep anything from getting in there. Unless you have a complication and your doc tells you not to, you can have sex until you give birth. As long as you are comfortable anyway!
answer #1 is right. your cervix is tightly closed so that no bacteria can get in and harm the baby. and its not like you can get pregnant again. you shouldnt have anything to worry about. but if you are really that concerned go talk to your doctor. i had sex all the way up till the day before i was induced and my bf never pulled out. no harm done.