One of the issues of an unrealistic goal is also motivation. Each individual must ask themselves if their goal motivates them and if so how does help them get started. In the example already listed, losing 100 pounds is a good goal, but to break it down into more realistic objectives measured week by week is far more motivational. If you set your initial goal at 5-7 pounds and reach that goal you will be far more motivated for the next week. Since we know this is a standard weight loss measurement from the first week of dieting and exercise, through water weight, then indeed this is building in some motivational success.
Men tend to lose more weight than women the first week and that also goes for the differential.
A man is more likely to lose the 7 and the woman more likely to lose 5. Allow yourself some cushion, or leeway in your estimates and goals. They must be motivational and concrete. Realistic goal setting includes the idea of whether or not the goal can be attained. Do you have to ability to attain that goal or is it outside your ability.