One issue that has grown more and more controversial in recent years is how and why "non-traditional" sexualities occur. Scientists have many theories on the issue, including the parental manipulation/kin selection, which involve the parents unconsciously producing a non-heterosexual child usually for the intent to assist in rearing a sibling's offspring. Ultimately, the question to be answered is if an individual can be born homosexual or what environmental factors lead to differing sexual orientations.
A study of twins in Sweden points to the problems of looking at only "Nature" or "Nurture" factors. Among mono-zygotic or identical twins there was a sizable, but far from high correlation between genes and sexual orientation compared to DZ or fraternal twins. Thus, the scientists who involved with the study concluded that there was not one side of the Nature vs. Nurture dichotomy solely responsible for sexual orientation, and it was, in fact, a combination of blurring of the two.
My reaction to any Nature vs. Nurture argument is that is rarely any one side of the issue that creates the individual. Rather, it is foolish to try to analyze issues through this dichotomy because the lines between them are so often blurred.