If you're using the same email address for posting questions on different sites, you should assume that it will be possible to associate them. We do not support opt-in "anonymous" posts in the way that (for instance) Slashdot allows you to post as "Anonymous Coward" while still using your normal account.
In fact, we do not actually support users operating multiple, separate accounts period. We don't prevent it, but if that's what you want to do then then the accounts have to actually be separate: different credentials, different email addresses, etc.
When you're posting as a "guest" (that is, not registering first), you're under no obligation to enter your real name or real email address... But if you do, you should assume that your post will be associated with you in some way. In fact, if you have existing accounts with the same email address the system may merge your new "unregistered" account into it and automatically link together your activity across sites.
...So, if you're using the same email address all over the network for posts you don't want associated, gravatar is the least of your worries.
Incidentally, the original idea behind Identicons (the pattern generated by Gravatar) was to enable forum users to reliably distinguish each other. Gravatar merely extends that concept to allow user-specified images, with the explicit intent of allowing your activity across multiple sites and services to be recognized. In other words, the "problem" you're describing is the whole point! If you don't like it, then upload your own image.