I, too, have something very similar happen to me after riding my mountain
bike. Rather than using the narrower, very firm racing-style seat that it came with, before i took it out of the bike shop I upgraded to a slightly wider and softer touring-style seat specifically designed for males. However, after riding for 30 minutes, I also get the numbness and tingling you describe for several minutes. It would have been much worse, I think, had I not changed out the seat.