Same-sex marriage sets a “dangerous precedent” which could lead to sibling marriage or polygamy, says Lord Carey.
Ahead of the Lords debate on the bill next week, the former archbishop of Canterbury argued there could be “unintended consequences”.
He has previously courted controversy by likening critics of gay marriage to persecuted Jews in Nazi Germany.
Gay rights group Stonewall said the comments were “regrettable” and accused the peer of “hyperbolic shroud waving”.
Lord Carey has been a vocal opponent of the government’s Marriage (Same Sex Couples) Bill for England and Wales, which will be debated by peers on Monday.
Keep stuffy old peers, politics and religion out of the bedroom.