I think the subject is simply not covered in any major Mahayana sutras -- gay sexuality is not found in tantric paintings either.
However, I found this reference :
http://www.reversespins.com/buddhismandgays.htmlPadma Sambhava had some pointed instruction for Yeshe Tsogyal on the subject of sex and homosexuality. The following quotes are taken from one of the most important termas ever transmitted by Guru Rinpoche or Padma Sambhava. It is known as the Dakini Teachings and it is a treasure map to the salvation of one's soul.
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"Sexual Misconduct ...
There are also occasions on which it is unsuitable to have intercourse even with your rightful companion. ...
3. It is unsuitable to have intercourse in an inappropriate orifice, such as engaging in the manner of animals.
Tsogyal, ... As before, the act of sexual misconduct is consummated by means of the four completing aspects, and again there are three types of results.
1. Through the result of ripening you will be reborn in the three lower realms. Even if you do take rebirth in the higher realms, you will have fights with your spouse and so forth.
2. The dominant result is that even in future lives your helpers, spouse, and so forth will be unresponsive and show various acts of ingrattitude.
3. The result corresponding to the cause is that your unwholesome habitual tendencies will cause you to take pleasure in sexual misconduct.
Tsogyal, if you give up these acts and refrain from engaging in them, you will obtain the opposites of their results, so abandoning them is of great importance. ..."
(Dakini Teachings, pp. 39-40)
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This is really not a part of any sutra.