It depends on the sect of Buddhism. Many would say to abstain from sex in general but I don't think homosexual sex is ever singled out as a particular issue of any sort-at least not that I have read.
In many Buddhisms-one tries to overcome any and all desire, and any other types of attachments-including to one's own body. They would say one is only finally released in death.
Personally I find that view erroneous. In my sect of Buddhism, there are no rules at all-the idea being that the pure philosophy would give rise to wisdom to allow one to act correctly, but from the heart in a true fashion as any behavior based on will power is insincere and false.
There are many gays and lesbians in my sect. I cannot think of when I've heard of sexuality being mentioned at all, much less targeting something at a particular kind of sexuality or preference. You are born alone and you die alone, so you are only accountable to yourself in Buddhism.
Now to get perhaps closer to the heart of the matter: There are no bad acts in Buddhism. An act has no merit by its own accord. What matters according to Buddhism is the NATURE of an act. This means that homosexual sex vs. hetero would have no difference by the accord of the relative acts involved there. What matters in Buddhism is cause and effect and a cause is determined by the nature of the cause-WHY did you do something? That is the issue. So like with any sex-the nature of it would matter, not the act itself. The nature is determined by what motivated you to do it. Selfishness? Love? you get the idea.
In my tradition (Zen), The Third Precept reads; "I resolve not to engage in improper sexuality, but to be caring and responsible." What is "improper"? Buddhism is founded upon the principle of ahimsa; "non-harming"; ubiquitous to the sutras are admonitions to refrain from harming any living being...including yourself. The "four abodes"; loving kindness, compassion, sympathetic joy, and equanimity are highly valued and encouraged. Therefore one can extrapolate that any sexual activity that is harmful; to others or oneself, would violate the Third Precept. I think of this in terms of exploitation; not just the common examples of rape or child molestation; but any relationship where one is indifferent to the needs of the other person, or attempting to get their needs met at the expense of another...essentially masturbating using the other person's body. Conversely, if it's a mutually "caring and responsible" relationship, then it's considered prima facie to be upholding the Third Precept; no questions asked. The sexual identity or preference of either party is completely irrelevant, and doesn't enter into the equation at all. Many in our sangha are LGBT, and I'm very glad that they feel accepted and comfortable enough to bring their mates to sangha gatherings as any other family, as every one of them are dearly beloved Dharma sisters and brothers. We would be so much less without them beside us.
"Venerable Hsing Yun, one of the premier figures in contemporary Chinese Buddhism, has stated that Buddhism should never teach intolerance toward homosexuality, and that people should expand their minds:
'Marriage is an institution that reflects the values of the society that supports it. If the people of a society no longer believe that it is important to be married, then there is no reason why they cannot change the institution of marriage. Marriage is a custom. Customs can always be changed. We can find the same core point in this question as we have in others — the ultimate truth of the matter is that individuals can and should decide for themselves what is right. As long as they are not violating others or breaking the laws of the society in which they are living, then they are free to do what they believe is right. It is not for me or anyone else to tell them that they must get married if they want to live together.
'That is their choice and their choice alone. The same analysis can be applied to homosexuality. People often ask me what I think about homosexuality. They wonder, is it right, is it wrong? The answer is, it is neither right nor wrong. It is just something that people do. If people are not harming each other, their private lives are their own business; we should be tolerant of them and not reject them. However, it will still take some time for the world to fully accept homosexuality. All of us must learn to tolerate the behavior of others. Just as we hope to expand our minds to include all of the universe, so we should also seek to expand our minds to include all of the many forms of human behavior.
Tolerance is a form of generosity and it is a form of wisdom. There is nothing anywhere in the Dharma that should ever lead anyone to become intolerant. Our goal as Buddhists is to learn to accept all kinds of people and to help all kinds of people discover the wisdom of the teachings of Shakyamuni Buddha.'" ~ Hsing Yun, Buddhism Pure and Simple
Years ago, I heard a story of a monk visiting the New England Aquarium in Boston. He'd look intently at each display, then tap on the glass. If he got the attention of some sea creature, he'd deeply bow, with great grace and dignity.
When I bow to you, I'm not bowing to the superficial you; the egoic tangle of desires, beliefs, prejudices, delusions, judgements, hopes, fears, confusions, certainties. I'm looking beneath that, behind that, beyond that, and bowing to your inherent divinity; your innate Buddha nature.
If you're a man called Bradley, I bow to you in all sincerity. If you're a woman called Chelsea, I bow to you in all sincerity.
I revere you and can never revile you, for you are the Buddha.