So, I've been trying to figure out why my TextField's were becoming multiline. Turns out that if GUILayout.TextField(string, int, GUIStyle) is used it is calling the internal DoTextField with multiline set to true instead of false. You can see this using ILSpy...
public static string TextField(string text, int maxLength, GUIStyle style, params GUILayoutOption[] options)
{
return GUILayout.DoTextField(text, maxLength, true, style, options);
}