I've done some research on this matter - though nothing recent. It seems that the X chromosme from the mother apparently leads to homosexuality (though this is balanced by the Y chromosome from the father which leads to heterosexuality!) In which case, given that humans by default have an amount of attraction from birth (neccessary for us to survive as a species, else we wouldn't trust our own gender nor would we have any aspirition to adopt their characteristics aka role models), you are not born with a definitive sexuality. You're given equal amonts of homo and hetero predisposition. Your upbringing/environment will ulimately sway you to 4 possible outcomes; hetero, homo, asexual and bi.

So technically speaking, we're both right.


edit: Sort of except I would place homosexuality as something one has a control over (again, this uses the definition of someone committing sodomy to be counted as a homosexual) whereas infertility and other diseases are something one does not have a choice with.