I just pull over on the side of the road.If you're wise and have not so much nerve, carry a bag with you, so you can dispose of it in a restaurant toilet.
The Volt, Odyssey and bike have a roll of toilet paper for emergencies. After a coffee spilled in my side luggage during the last ride and ruined the TP and book @shanekingsley gave me I put the TP into a waterproof bag!