Just read the first and last pages so I'm sorry if this solution has already been offered.
Years ago I used to average 250 - 300 miles a week on a bike, year in and year out and never had that problem.
You need to be aware of a number of small but important issues.
i. The saddle should be level or as near as dammit.
ii.Wear proper cycling shorts.
iii. Use a properly shaped and narrow saddle. It's like a cheese wire att first but you get used to it.
iv. It's vital that you get your weight distribution right. Approximately one third on each of the bars, pedals and saddle.
If you do nothing else, make sure iv. is right.
JJ