Explanation:

I also had the same issue with my members Profile. It was beeing updated every time some one visit members profile. I used a System.DateTime type Pofile field. This field is used to save the member birth date. If your ASP.NET 2.0 user profile’s LastActivityDate gets updated every time you use Profile.GetProfile() it’s most likely the result of having complex types defined in the profile. Here’s an excerpt from QuickStart Tutorial page:

The Profile feature will automatically determine whether or not a Profile is dirty. If the Profile appears to be dirty, the ProfileModule that runs at the end of each page request will call the Save method on the Profile, thus saving data using the configured provider(s). However, the Profile feature can only reliably detect changes to data when the data is typed as either a System.String, or as a primitive type like System.Int16, System.Int32, etc… If a Profile contains more complex data-types, the default behavior of the Profile feature assumes the Profile is dirty, and will always attempt to save the data. To optimize performance, a developer can implement logic in their pages to determine if the Profile really is dirty. If a developer determines the Profile has not changed, they can hook the ProfileAutoSaving event by writing an event handler in global.asax. The event argument includes a property called ContinueWithProfileAutoSave. If a developer sets this property to false, then ProfileModule will not attempt to save the Profile.

Note that if a developer never wants the ProfileModule to attempt saves, the automatic save behavior for the feature can be turned off by setting the automaticSaveEnabled attribute on the element to false.