Yesterday I "complained" about some unexpected behaviour in .TEXT - but as it turns out, stupid is as stupid does...
Apparantly I had chosen to ignore the option to include PostConfig enum(s) when creating a new post. The PostConfig enum allows you to define which of the Advanced Options should be "on":
net.geekswithblogs.PostConfig p = net.geekswithblogs.PostConfig.IsActive|net.geekswithblogs.PostConfig.DisplayOnHomePage|
net.geekswithblogs.PostConfig.IncludeInMainSyndication|net.geekswithblogs.PostConfig.AllowComments;
This takes care of it - notice the absence of net.geekswithblogs.PostConfig.IsAggregated (which would include the posting in the aggregate feed).