Riding will not cause hemorroids, but will certainly aggravate them. To deal with the problem do two things - avoid internal and external irritation.

Internal irritation is normally caused by eating roughage (popcorn, nuts, etc.). Deal with this by using a stool softener after each meal. Metamucil is a dandy one because it is made of psyllium seeds that not only provide bulk but also leach fat from your system.

External irritation is normally caused by excess scrubbing after bowel movements. Deal with this by shaving the external areas and doing less scrubbing.

Between these fixes, you'll be able to ride pain-free.