What's wrong with you people? Ever hear of demographics? The statistical data of a population? Pretty routinely asked in surveys. Could it possibly be why they ask sexual orientation? Nah.
As far as wanting us employees to participate in the survey, yet not caring what your responses are...you seem to be putting the cart in front of the horse. They need to get participation in order for the survey to "count", so yeah, they want participation first. The results are what they are, but pointless if no one participates.
Of course this just seems natural to me, as I am not some conspiracy theorist.
Work, do my job, take the survey or not, donate to UW or not, whatever.
It takes all kinds, I suppose.