Google "spies" on everyone who uses any Google service, and likely any Google product; that's exactly their business model. They sell ad targeting based on your Internet behavior, and a big part of their marketing advantage is that they own a lot of different sites that coordinate their records of your behavior.
The real question is how much to trust them to anonymize things, assuming that the information they gather and collate about you---your likely demographic properties (age, race, sex, occupation, geograhic region of residence, sexual orientation, political leanings, health issues, etc.)---is not in some way passed on to advertisers---or implicitly to people who monitor what ads you're served, without Google having to cooperate.
Don't expect actual privacy on the Internet. Assume that there are data miners out there who notice what ads you are served, and what that indirectly reveals about you, even if Google is not just handing over explicit information about you to the US government in response to secret FISA court orders. Assume that spies know more about you than your spouse does---who you've called at what hours, who you've chatted with, what you've discussed, etc.
Never answer a question on the Internet truthfully, or consistently. "Polls" on websites are mainly a way of targeting you, and selling information about you to advertisers, or to anyone who'll pay for that information, e.g., spy agencies. If you're a conservative, answer liberally half the time, and vice versa. Don't give your real age, or give your real sex more than half the time. Make them work for information about you, and make sure their data sets are full of noise.
Of course, if you do these things---e.g., being inconsistent in answers about your age and sex---that will set off alarms and draw scrutiny.
Welcome to the Kafkaesque 21st century.