The answer is yes, you can physically have sex with a girl while she is on her period.

However, not all couples like to have sex during a period.

It all depends on the couple and how they feel about it.  Some girls (and guys) don’t like the idea of having sex during a period.  It can be messy and uncomfortable.  Some girls don’t feel good and can have cramps and other symptoms when they have their periods. 

Some couples see a period as a good time to take a break from sex, while others are totally comfortable with it.  Whatever you decide, you should talk about it with your partner to make sure you are both comfortable. 

Remember though, if you have sex with a girl during her period, she can still get pregnant.  Because some periods last longer than a week, a girl’s ovulation can overlap with her menstruation, and you won’t know when this is happening. 

Also, sperm can live for 3-5 days inside a woman’s body.  So even if your partner ovulates a day or two after having sex, she can still get pregnant.  Whenever you have sex, you have the change of getting the girl pregnant—so you always need to use birth control, even when a girl is having her period.

And both guys and girls can get sexually transmitted diseases when they have unprotected sex even during a period.  In fact, having sex when a girl is having her period can make it easier to spread some STDs and your risk of getting an infection is higher. 

Every time you have sex you need to use a condom so that you and your partner can be protected from pregnancy and sexually transmitted diseases.