I could bring up papers in which the GnRH and various other hormones are included, but yes, up to an extend, it has to do with the "abnormal" instability of male and female hormones triggered by the brain (even if the kidney is a bit responsible too). Not totally based on genetics though, but as everything else, our behaviour and tendencies are based on our personal experiences from an early age too. A mix of hormone balance, experience, and many other factors we don't know up to which point they add to this, leads to a person to be attracted to one gender, or another, or both, with all 3 categories having fluctuations or not the same type or intensity of attraction. (sorry I can't explain it very well). Just find the person who makes you emit more Oxytocin than usual.
If I may, I shall share some of my views on the matter.
First of all, as far as the legal document is concerned, which is the main focus of the discussion, the marriage is a connection between two people, which verifies their bond, and seals it in front of their peers and the community. Marriage, as a procedure, is not only an emotional bonding, but also a safety lock in terms of legal procedures that are applied in each respective country and state inside of it. That said, two homosexual people can now by the law verify this connection and are provided with safety when a situation of a divorce, death, social services, legal documents and taxation (with all the benefits married couples enjoy). Therefore from a strict legal point of view this is a landmark.
At the same time, the UN has declared several human rights that supposedly apply to all those considered human, regardless of sex, sexual orientation, race (as in skin colour), any disability, and many other less relevant categories. The right of dignity is first on the list, followed by the right of life. This hierarchy proves that a life in which certain civil rights are not given to a group of people, based on their personal views or actions (which of course do not cause harm to the others, as one's freedom stops where another's begins) is against the very principle of dignity, even if... technically it's still life. Hence I can only consider it arbitrary when rights are denied based on personal preferences or characteristics that are neither up to change or negotiation, nor cause harm to other groups simply by existing. Allowing those people to marry, in terms of ethics and civil rights, is then the only logical action.
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