Don't get the request stream, quite simply. GET requests don't usually have bodies (even though it's not technically prohibited by HTTP) and WebRequest doesn't support it - but that's what calling GetRequestStream is for, providing body data for the request.
Given that you're trying to read from the stream, it looks to me like you actually want to get the response and read the response stream from that:
WebRequest request = WebRequest.Create(get.AbsoluteUri + args);
request.Method = "GET";
using (WebResponse response = request.GetResponse())
{
using (Stream stream = response.GetResponseStream())
{
XmlTextReader reader = new XmlTextReader(stream);
...
}
}