I have found a dance belt great for riding: both for protection and for aesthetics if wearing tight jodhpurs or breeches. You need to make sure your genitals are pulled up from out between your legs: this will stop them from getting caught between you and the saddle (ouch!), particularly at trot.
I wear a dans-ez belt (size L): not sure what you mean by a seam behind the genitals, but it's comfortable. You say your crotch rests on the saddle? Some people say that you should be sitting on your seat-bones and that your saddle must be too small or your seat is too forward. Classical dressage folks would disagree - in the three-point seat, the crotch is the front of the seat. This alone is a justification for using a dance belt, although not at the expense of developing a firm, controlled seat overall.
Hope this helps.
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