There are a few weird differences between opposite-sex marriage, same-sex marriage, and civil partnerships. In particular, in the grounds on which the partnerships can be annulled on the application of one partner.

The first is that same-sex marriages aren't voidable for reasons of non-consummation; basically it used to mean "PIV sex" and Parliament didn't fancy trying to define consummation for same-sex couples in legislation.

The stranger one is that only marriage is voidable for reasons of venereal disease.