"True, all things are clean, but it is injurious to the man who with an occasion for stumbling eats. It is well not to eat flesh or to drink wine or do anything over which your brother stumbles"
I always hated those people that always had to interveen in your activities to say, "You may want to be careful others don't get stumbled because of this." I would just say, "If my doing something as simple as this stumbles them then their faith isn't very strong to begin with and if they stumble it won't be my fault. They should worry about themselves. Maybe you should spend more of your time strengthening him/her faith so that stupid things like this won't stumble them to begin with."