Let them be only thine own, and not strangers' with thee. By confining yourself to chaste intercourse with your lawful wife, be assured that your offspring is your own. Promiscuous and unlawful intercourse throws doubt upon the paternity of children.
Thy children may be thine, they may belong to another. The natural pride which is felt in a legitimate offspring is the motive put forward to commend the husband to confine himself exclusively to his wife.
Grotius on this verse remarks, "Ibi sere ubi prolem metas"—"Sow there where you may reap an offspring." Them; i.e. the children referred to figuratively in the preceding verse, from which the subject of this verse is supplied.
The repetition of the pronoun which occurs in the original, "let them belong to thee, to thee," is emphatic, and exclusive of others. The latter clause of the verse, "and not strangers' with thee," covers the whole ground.
The idea of their being strangers' is repulsive, and so gives further point to the exhortation.