Is Buddha a Man or Woman?
Was Buddha a Man or Woman?
Was he/she sometimes know as a man and sometimes as a woman?
Why was he/she sometimes know as a man and sometimes as a woman?
What was his/her real gender?
Give proof please.
He was a guy. What makes you think anyone thinks differently? Are you referring to celestial Buddhas or the historical Buddha? Are you confounding God (other religions) with the Buddha (a religion rather divergent from Christianity)?
May be a duplicate of http://buddhism.stackexchange.com/questions/3322/can-the-buddha-ever-be-a-woman
You said "is Buddha" (using the present tense, not "was"), so does that mean you are not asking about the historical Buddha? It's difficult to be sure of what you are asking about. Is your question related to Why does the Buddha appear androgynous in some depictions? Or to Andrei's answer about gender?
The answer to this question has a lot of things that change the answer, depending on whom you ask. It would depend on which Buddha we're talking about, which lineage, which region, which culture, and which set of histories we're willing to accept as historical fact. As such, there is no singular answer which could possibly suffice.
The Buddha, historically, was male: his name was Siddhartha Gautama and his title was Shakyamuni because he was a prince and later king of his tribe in Nepal. He called himself "Buddha" because it connotes being awakened by light. As such, anyone who has reached this level of personal and spiritual evolution is considered to be "a Buddha".
In some lineages, female Buddhas exist; in most, all Buddhas are required to be male.
In some cultures, the mere idea of a female Buddha is offensive; in others, the idea is acceptable; and in still others, the idea that anyone has a problem with that at all is offensive.
In some histories, there is acceptance of the story of Guanyin (also called Kwan Yin), who is sometimes called the "Medicine Buddha". But she is also considered a goddess of healing and a Taoist immortal, among a few other highly-regarded titles.
One of the teachings of the Buddha is to overcome preconceptions. If we're bound to the idea that only men can become Buddha, it would make sense that the Buddha might reincarnate as a female to try to teach this idea, perhaps after being a male who seeds the idea.
Again, there are so many, many ideas out there about this that there is no singular correct answer. And so to answer, I would just write: "it's not outside of the realm of possibilities, either way".
"Old Buddhism" meaning the original Buddha? Actually, there was no "Buddhism" until more than a century after the Buddha's death... it was merely a branch of Hindu philosophy, and the Buddha was a highly-regarded swami and a guru. There was never an intent for separation on the part of Gautama.