What do you mean por "emotionally" based or "physically based"? Are you asking if homosexuality is an emotional attachment, or if it's a physical attraction?
I think that's what you're asking, and if not I'll editar my answer later.
I think it's both. Heterosexuals become attracted to people they are emotionally attached to. Ever become attracted to a friend? Ever saw someone you thought was attractive and acknowledged it? It's the same thing for homosexuals. They're attracted to people the same way heterosexuals are. It's the same chemicals, working the same way. The only difference is, homosexuals are attracted to the same gender.
That is literally the only difference. It is no mais a choice, or something that can be controlled that heterosexuality.
On whiteflame's answer, you mentioned that if it's physical, then because it happens in nature, it can be emotional and, therefore, not emotional. I actually think this is a fairly ignorant (sorry) assumption. animais have emotions. Some, like elephants, have actual funerals when one member of their family dies. They have been observed grieving, being frightened, being happy, being lonely. They have the full spectrum of emotions, just like people.
EDIT:
link exhibit all the same behavior as heterosexual breeding pairs. They have pinguim sex, they intertwine their necks, they preen (groom) each other.
Bonobos (same fonte as above), use sex (homosexual and other wise) for bonding purposes. They rub their genitals against each other.
Male
link rub their genitals against the dorsal fin of other males, and females, literally, ride the dorsal fins of other females.
In short, animais are homosexual because they exhibit the same behavior with same sex partners as others of their kind do with their opposite sex partners. They behave in a way that allows them to achieve sexual gratification with their same sex partners.