Why can't you have s3x while on your period?!

There is no reason that you can't have sex while you are on a period. Some women do not lime the thought of it,some men are completely turned off by the thought of it however some women find that the week they have their period is the week they feel at their most randiest.
It is perfectly safe I would just advise not doing it on the heaviest days of your cycle and use a towel to avoid any mess.
Also make sure your partner knows and agrees to it before you begin to avoid embarrassment half way through.

Physically there is no reason. It often helps with cramps, its just a little messy.

If JW stands for Jehovah's Witness and not Just Wondering, it is is because the bible says that blood is scared and if you go back to the old testament any man or woman knowingly having intercourse while the woman was menstruating was put to death. So obviously it was considered a big no no. If it was discovered after and they didn't know that she was bleeding then they were both considered unclean and would be isolated for a week and he would have to bathe daily to cleanse himself. any woman who was menstruating was considered unclean and was to be isolated. Also anything she touched was considered unclean as well and had to be purified. You can look up the references in the back of your bible in the index.

Edit: I'm not saying that they live by the law of moses or that JW's practice those things. That was simply the law that God gave to the Israelites. But that law was the basis for the reasoning that God would not approve of intercourse during menstruation now. Because of how strict the laws were about menstruation at that time it shows that it was not something taken lightly and even though now they are under the law of love, during menses intercourse is to be avoided to show respect for the sanctity of blood in god's eyes.

That was the answer I was given a decade ago by an elder. And also what was told to my girlfriends before they got married.

I do not agree with Genexsis about JWs. They do not live by the law of Moses but the law of Christ and are not critical about woman being in her period or not. Just like Christ who did not judge the Woman with the issue of Blood who touched his Garment and got well. JWs live, eat and stays with their wives even when they may be in their periods. I think it is those who do not know that the law of Moses stoped when Jesus was nailed to the Stake Romans 10:4.