Even if you have honest answers, it's a minefield to phrase questions so that you get the data that you think you are getting. For example, there's a statistic

Even if you have honest answers, it's a minefield to phrase questions so that you get the data that you think you are getting. For example, there's a statistically significant amount of men that have sex with other men, but who self-identify as heterosexual. That group might end up in either camp if the survey isn't aware of the issue and phrases the questions badly, which makes quantitative studies much harder to do.