Broadly speaking, yes. The European Convention on Human Rights (ECHR) Art 14 prohibits discrimination in relation to Convention rights such as the right to marry in Art 16.

The prohibited degrees of relationship for marriage prevent genetic birth defects arising from sexual intercourse between persons who are too closely related. Since that applies to heterosexual spouses only there's no need to apply the same rules to same sex partners. Doing so unfairly discriminates against those who fall within the prohibited degrees so you finish up with a situation where some same sex partners can get married and others can't which will contravene Arts 14 and 16.

To get the Bill through Parliament in its present form Cameron will have to admit that he intends to derogate from those Articles and discriminate against some same sex partners.

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