Singer warns not to consider moral positions as "rules" as stated in this essay, but guidelines. You can't be 100% perfect at all times (as Franklin discovered).

In addidition, moral views are mutable. My friend has been vegetarian for 10 years and on account of some book about a farm in Virginia he thinks maybe eating meat is all right. My argument against his decision may have some weight, but ultimately virtue and ethics are an individual's business.