I don't know about the roads where you are at but unless they are particularly oil coated

I'd say it is your bike. I used to get some oil on my pants from my knee area rubbing on my NH. I have none from the scooter or the Bonnie.
If you're getting black riding pants you really aren't going to see oil. You could scotch guard them to help release dirt better when you wash them.
I wash my gear pretty regular. I use shampoo to pre-spot (works well on grease) wash cold and hang dry. The fabrics used to make riding gear release oil pretty well, not like cotton jeans.