It has been mentioned by others in the past, and in reading material regarding anal play and sexual activity, that difficult-to-digest materials, like seeds, nuts and certain vegetables, can prove challenging when it comes to anal play.
Think about seeds, for a moment. Many seeds are carried far from their original plant by animals that eat them. The seeds themselves must have the proper defense mechanisms against the animals' digestive tracts. The outer shell of an intact seed is surprisingly resilient!
While it can be unavoidable in processed foods garnished with seeds, in your own cooking and baking, there are solutions. If you still want the health benefits of various seeds, such as flax, you can grind them up in a food processor until they're a fine meal.
I tend to eat a good deal of nuts and seeds myself, but I only rarely encounter any issues. While it may not eliminate the risk altogether, a bulk-forming fibre supplement, such as original Metamucil, should help you to evacuate everything effectively.
Summary, because this post was too long:
Animals poop seeds. It's the circle of life!
When making food, grind seeds into meal.
To help eliminate seeds and debris, take bulk-forming fibre supplements.