It depends on which book you read and which version of the bible you have as to the exact numbering of the commandments. And Catholics (amongst others) take the reading of the Bible far too literally and often in a completely dissociated manner to modern society. Abstinence prior to marriage is not an express commandment. It may be implied and even stated in other parts of the Bible, however I do not see it as an express commandment as the commandments are there as a guide to honour the Lord and each other. I would argue that there can be no better way to honour an individual than by making love.
But back to the point, I was 16.