What Are The Main Scientifically Accepted Theories About The Origin Of Homosexuality And Lesbianism?

Hmmm... Well, first I'd like to congratulate you on having asked such a question in such a completely non-oriented fashion as to fully obscure the reason for the question. Well done.
However, I have some bad news. Currently are no simple "...main scientifically accepted theories about the origin of Homosexuality...".
While there is considerable conjecture, and innumerable hypotheses about the origin of human sexual orientation, there are no fully accepted theories that are considered the be even so much as 'mostly true'... There seems to be a fair amount of agreement that there is definitely a genetic element that makes the 'proclivity' a possibility (fully supported, as far as I'm concerned, by the appearance of such behavior among other creatures besides man), but no one has been able to point to a single gene or genetic signature as definitively 'the gay gene'.
For the most part, the consensus seems slanted toward the idea of genetics playing a part in the development of homosexual orientation, there also seems to be definite influence by such things as family relations, social stresses, reinforced behavioral choices at crucial phases of development, even 'intrauterine' (prenatal) influences that eventually lead to such orientations.
So the simple answer is that... Well, we really don't know. However, there can be no legitimate claim of homosexual orientation being 'unnatural' in any way, as science, and zoology have long documented such inclinations in nearly all (if not all) other species. Thus it is just as natural as heterosexual orientation (just usually not so genetically successful, as fully expressed homosexuality removes those so inclined from the majority of reproductive opportunities).
I hope I've been of some help in answering your question (as much as I could anyway), and if so, please rate the answer appropriately. Thank you, and good luck.