If you want some more fun with the subject, check out Hanne Blank's Straight which argues that the identity of heterosexual is a fairly recent thing-- only about a century old, as I recall. Previously, people thought in terms of sexual behaviors, not identities.

To some extent probably it is: the gay identity historically arose as a reaction against the previous negative view of homosexuals as people affected by a mental disease. Indeed the word "gay" was chosen specifically to avoid and reverse the negative connotations of "homosexual".

To some other extent, it is probably be a result of Western societies becoming more wealthy, democratic and individualistic, therefore individuals feel more free to follow their preferences rather than social expectations of their family/clan/state.

I don't think that's necessarily implied.

Obligate homosexuals probably always existed, we just can't be sure if it was at the same relatively high rates as today. But only recently have they organized socially and politically to demand equal rights. As part of this movement, homosexuality became an important part of their identity, and formed a group identity, and so the social and psychological character of how people express their own homosexuality changed. But that doesn't mean the core features of being attracted to people of the same gender, and not attracted and unwilling to have sex with members of the opposite gender, changed.

I see this as similar to the historical emergence of nation-states. A medieval peasant didn't consider being French an important part of who they were, didn't have a French citizenship. But they still lived in France and spoke French; in that sense there were Frenchmen then just as today.