According to this document (page 12):
Table 1 indicates that, of the 30 EU/EEA countries, five – Belgium, France, Portugal, Slovenia and Spain – have introduced quotas by law that are binding for all political parties.
The Equal Opportunity principle states that:
Equal opportunity is a stipulation that all people should be treated similarly, unhampered by artificial barriers or prejudices or preferences, except when particular distinctions can be explicitly justified. [...] not go to persons for arbitrary or irrelevant reasons, such as circumstances of birth, upbringing, having well-connected relatives or friends, religion, sex, ethnicity, race, caste, or involuntary personal attributes such as disability, age, gender identity, or sexual orientation.
A legal form of this principle is The Equality Act 2010 which
prohibits discrimination in employment or in the provision of training and education on the grounds of any of the following protected characteristics: age, disability, gender reassignment, marriage and civil partnership, pregnancy and maternity, race, religion or belief, sex, and sexual orientation.
Question: how comes that electoral gender quotas systems apparently break the Equal Opportunity principle?
Important note: This question is not about pros/cons of Electoral Gender Quota Systems which are detailed in this document at page 18 and are off-topic for Politics.SE.