Not to nit pick, but the Bible didn't use the qualifier 'adult' man and woman. It is common knowledge that females have been reproducing from breeding age since the world began. It is only modern sensibilities that frames the limit of 18 and correlates 'adult' behavior with it. Even in the 20th century, it was quite common for young girls to marry at 14-16 and already have several kids by the time they were 20.
This whole topic gets to be ridiculous, especially with modern 'religious' types who live in a bubble and have no experience in the rest of the world. Though I will affirm that sex should be reserved for a married or monogamous man and woman.