A man with a wife and family cannot be so adventurous, or take so many chances, as an unmarried man. A hostage is a person given to another as a pledge or security. ‘Hostages to fortune’ are persons or things that one may lose.
The proverb comes from Francis Bacon’s essay, Of Marriage and Single Life: ‘He that hath wife and children hath given hostages to fortune; for thay are impediments to great enterprises, either of virtue or mischief.’