To my knowledge, there are no teachings in the suttas that refer to sex outside of marriage as part of the Buddhist path. Every teaching about sex I have read is referred to in the context of marriage (e.g. AN 4.55; AN 4.53). Any sex outside of marriage is obviously motivated solely by lust (rather than mostly by compassion), which is unwholesome according to the suttas (MN 9, which states lust is the root of the unwholesome). Therefore, it is quite logical that any sex outside of marriage is a volition of the precept because it leads to hungry ghost, animal (AN 2.9) & hell realms (DN 31). Do we think a person that develops a habit towards unmarried uncommitted sex won't become a hungry ghost; similar to how men (who for some reason post on Buddhist chat sites) are often addicted to pornography?
DN 31 says parents show compassion & protection (security & safety) towards their children by arranging a suitable marriage. This is the meaning of "protected by parents"; that parents protect their children from becoming hungry ghosts & bringing shame upon the integrity of the family. Thus, that the children are married means they have been protected by their parents, regardless of where they live. For example, in the time of the Buddha, generally the married children lived in the house of the husband's parents. DN 31 says:
In five ways, young householder, a child should minister to his parents as the East:
(i) Having supported me I shall support them, (ii) I shall do their duties, (iii) I shall keep the family tradition, (iv) I shall make
myself worthy of my inheritance, (v) furthermore I shall offer alms in
honor of my departed relatives.
In five ways, young householder, the parents thus ministered to as the East by their children, show their compassion:
(i) they restrain them from evil, (ii) they encourage them to do good, (iii) they train them for a profession, (iv) they arrange
a suitable marriage, (v) at the proper time they hand over their
inheritance to them.
In these five ways do children minister to their parents as the East and the parents show their compassion to their children. Thus is the
East covered by them and made safe and secure.