Contrary to popular belief, that "bigger is better," oral sex, anal sex, or vaginal intercourse with a well-endowed man can be challenging. For some, the "gag reflex" makes it difficult to have the entire shaft of the penis in one's mouth. For others, penetration causes pain.
GoAskAlice - University of Columbia - Support Forum - Sexuality - Mens Health

Often Viagra does not work because of improper use: if appropriate the highest strength should be used, the pill taken on an empty stomach with minimal alcohol use, taken at least one hour before use, and sexual stimulation is needed.
Irwin GoldStein, MD - Center for Sexual Medicine at Boston University School of Medicine