As homosexuality is not explicitly mentioned in any of the Buddha's discourses (more than 20 volumes in the Pali Text Society's English translation), we can only assume that it is meant to be evaluated in the same way that heterosexuality is. And indeed it seems that this is why it is not specifically mentioned. In the case of the lay man and woman where there is mutual consent, where adultery is not involved and where the sexual act is an expression of love, respect, loyalty and warmth, it would not be breaking the third Precept. And it is the same when the two people are of the same gender. Likewise promiscuity, license and the disregard for the feelings of others would make a sexual act unskillful whether it be heterosexual or homosexual. All the principles we would use to evaluate a heterosexual relationship we would also use to evaluate a homosexual one. In Buddhism we could say that it is not the object of one's sexual desire that determines whether a sexual act is unskillful or not, but rather the quality of the emotions and intentions involved.
FOR FURTHER READING...http://www.buddhanet.net/homosexu.htm