>> Article 14 – Prohibition of discrimination:
>> The enjoyment of the rights and freedoms set forth in this Convention shall be secured without discrimination on any ground such as sex, race, colour, language, religion, political or other opinion, national or social origin, association with a national minority, property, birth or other status. [0]
Granted, its not about marriage, but according to the ECHR, its a human right to live free of discrimination based on sexual orientation.
[0]: https://www.coe.int/en/web/conventions/full-list/-/conventio...