My friend tells me it depends on how you view homosexuality. He says if you see it as a belief/religion, then yes. Otherwise no. What do you think?
You can discuss philosophically the nature of sexuality, and homosexuality in particular. Also, I don't see at all how homosexuality is a belief or a religion (or both). We can be utterly straightforward and look up what the word means in the dictionary: Merriam-Webster defines 'homosexuality' as 'the quality or state of being homosexual'. Can a quality be a belief or a religion? I don't think so. However, 'homosexuality is not unnatural', for example, is a perfectly legitimate belief (I can't think of a homosexuality-based religion, sorry :) ).
EDIT: take a look at this SEP article on the subject, it also contains a huge list of references to works of actual philosophers.
You can...but that depends on what about homosexuality you are discussing.
Are you discussing whether it is morally 'right' or 'wrong' to be a homosexual?
Are you discussing the idea of homosexuality as a philosophy in and of itself?
Are you discussing the very concept of whether or not homosexuality has any philosophical bearing at all?
Homosexuality is a broad concept, and has plenty of areas of conflict that can be discussed from a philosophical standpoint. Its very nature, its impact on society, whether or not it is a choice, what that would mean for society and individuals, all are ripe concepts for discussion.
Homosexuality itself is a very, very broad topic, and 'discussing it philosophically' would be akin to discussing a religion, an ethnicity, a moral viewpoint, or any idea - you can do it, but without context or a direction to discuss it in, there isn't much point.