Internet or mutual friends of some sort. Also, they may meet at a gay bar or gay parties, etc...
In the case of humans, a bisexual would be anybody whom carry a desire (sexual or emotional) for both men and women. Bisexual and homosexual characteristics have been seen in both humans and animals...
Without more information, it's kind of hard to tell you.
Bisexuality is the emotional or sexual desire for both men and women.
"She" may be bisexual because that's the way she personally feels. You can't help your feelings. And you shouldn't judge somebody for that.
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