The anti-gay position is mostly based on religious and political beliefs fueled by long held cultural bias.
Religious and political dogma if it goes unchallenged eventually becomes truth.
In Christianity, Islam, and the Jewish tradition, to name a few, slavery was justified, the treatment of women justified, the persecution of the Jews and the continuing persecution of homosexuality are or were all justified by their religion.
Religion provides a license to indulge our prejudices and bigotry.
The opposite of the Anti-gay position is not the pro-gay position but the pro-humanity position