What is the attitude toward homosexuality in Buddhism?
I think that the reason you only find references to homosexual activity in the vinaya is BECAUSE the vinaya is the only place where there is a need to be specific regarding sexual acts... Oral sex is still oral sex whether it's performed by a man or a woman, an animal or even yourself.

In the Suttas, the teachings, it doesn't matter if you are attracted to the opposite sex, same sex, etc. it's all about craving,clinging, desire, attachment, and aversion all underpinned by ignorance. These come from the mind and go far deeper then sexual identity and attraction. 

As for Homosexuality and the Theravada tradition, buddhanet.net strikes again:

http://www.buddhanet.net/homosexu.htm


  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.