The summary posted hear doesn't say they "started off with" homosexuals, or that they asked them only one, possibly tendentious, question ("Were you abused?")
It says they started off with about 13,000 people surveyed between 2003 and 2006. 98% of the participants in the study identified themselves as heterosexual. It also doesn't mention having any "results in mind before the study was started." It would surprise me if they were not asked a whole lot of survey questions, and these particular correlations were found to be statistically significant.
But possibly you have read the study itself, and not just this very brief press report? I'm interested. Do you have a link?
The title of the published paper says a lot. "Multiple Aspects of Sexual Orientation: Prevalence and Sociodemographic Correlates in a New Zealand National Survey ". The key phrase being "Sociodemographic Correlates in a New Zealand National Survey". The author of the paper took a New Zealand study of sexuality and extrapolated conclusions. May sound bigoted but I have seen enough of these studies over the last 45 years to suspect they have an ulterior motive and that they sometimes "shop around" for datum that correlate their desired result. I simply don't trust them anymore. Kinsey was the first I read and look at how screwed up his results were.
My criticism of the conclusions were in comparison to my real-world experiences which significantly differ with their extrapolated results. I am not pro-homosexual or anti-homosexual. I don't have a dog in the fight until my rights are tread upon.