Why can’t you use adultery as grounds to end a same sex marriage or civil partnership?

20 October 2025 | Civil Partnership Law |

Because current UK law defines adultery as being sex only between a man and a woman, this does not usually apply in cases of infidelity within a same sex marriage or civil partnership. Instead, infidelity between same sex partners has to come under the grounds of unreasonable behaviour.

If you are in a same sex marriage and your spouse has sex with someone of the opposite gender, this will be classed as adultery, however, in civil partnerships there is no provision for adultery at all, so this would still be classed as unreasonable behaviour.