Given that studies of sexuality don't 100% declare that heterosexuality is genetic and may in fact be a myriad of different pre-natal, hormonal and genetic flavours, should heterosexuality be considered a legitimate expression of human sexuality?
Heterosexuality is an absurd term, the correct term is normal sexuality. Any sexual attraction outside of man-woman attraction has no natural purpose, and is therefore biologically abnormal as is usually the bi-product of genetic problems.
Oh right, so sexual attraction, and intercourse, between a brother and a sister is considered "natural" and biologically "normal"?
