Do you have to wash after sex? Yup. Jews have the same rules, and so would Christians if they followed Old Testament teachings. Here's the verse, from Leviticus 15:
18 The woman also with whom man shall lie with seed of copulation, they shall both BATHE themselves in water, and be unclean until the even.
One cannot pray or enter the temple while unclean. So, this is not all that surprising, is it?
All I can say is, thank God for Jesus. We are free from the curse of the law!