It really depends on my mood, but if the person is insistent, I will calmly look them in the face and say I was trying not to talk about gross things, but I have Crohn's Disease and uncontrollable bowl movements is part of the disease, so I purposely select an aisle close to the bathroom so if I need to go, I can go easily without soiling myself.

That always stops them dead in their tracks and at that point they don't want to talk to me anymore. A few will say I am sorry I didn't know you had a specific reason.

One person said, you could have just said I have a medical reason.

If you (the passenger looking to swap seats) are going to push and push and not take no for answer, I am going to tell you why I purposely chose my seat.