No, that's one extreme, the other extreme is to say if you have sex with someone who is 17 years 364 days old, it's rape, but the next day, it's consensual sex. You can't say that is rational.
Many people seem to have lost the ability to think rationally on this topic.
If a "blue lagoon" scenario happened, do you really think the two teens would wait around till their 18th birthday's to have sex? There is no magical metamorphisis that takes place on that date.