You were told that this would happen, and you agreed to it. You just didn't read what it said, as the above says.
Anyway, it's no reason to worry. All web-based email systems scan your email. It's all electronic, with nothing stored for future. Google just uses that information as they scan your messages to send keywords to their advert system.
Which would you prefer: adverts about dating agencies or other random things, or adverts for services and shops you might actually be interested in?