If you are looking for a definitive answer to this question, you are definitely going to be annoyed with me. As is the case with so many questions about sexual encounters and behaviors, the answer is that it depends.
Physically, it is absolutely possible to have sex on your period. There are even some awesome benefits to getting busy at that time of the month.
But before we get to the fun stuff, let's clear up some questions and make sure we are all on the same page.
1) Your period won't keep you from getting pregnant.
Being on your period does NOT mean you are protected against pregnancy and it certainly doesn't protect against STIs. Even though menstruation is the process of shedding your uterine lining, it is still possible for sperm to fertilize an egg and begin a pregnancy during your period.
Those sperm are resilient little fellows and can live within the vagina for up to FIVE days. Depending on the length and timing of your cycle, you may ovulate (release an egg on its journey from the ovary through the fallopian tubes and into the uterus) within five days of the end of your period.
That egg can be fertilized and implant in the new lining of your uterus, marking the beginning of a pregnancy. So remember to use protection if you choose to have sex.
2) Just because you CAN you have sex on your period doesn't mean you necessarily WANT to.
The most important question is not CAN you have sex, but do you WANT to engage in hanky-panky on the rag. If you or your partner are squeamish about the idea of blood, no one should feel pressured to engage in anything they aren't comfortable with.
Furthermore, the surge of hormones that is associated with menstruation causes some women to feel bloated, emotional, achy and crampy, or just generally disinterested in sex, at least for part of your cycle.
If this is the case for you, just because period-sex is an option, certainly doesn't mean you HAVE to try it. Talk with your partner about what is going on with your body and your mind. They should respect your decisions.
3) Sex doesn't have to mean vaginal intercourse.