Sexual arousal from seeing or touching the buttocks of another person. The term is not gender-specific. A person who is thus aroused is a "pygophilist" or "pygophile"; "pygophiliac" may be used if the arousal could be considered abnormal, pathological or extreme. In a finer sense, "pygophilist" can be used to infer a person whose predilection includes actively touching buttocks, as the suffix -ist connotes "one who does", whereas "phygophile" (-phile, "one who likes) could include those who are content merely to look at buttocks.
Pygophilia is not that uncommon - my best friend is a total pygophilist - but that guy at the office has had his hands on more ass than a donkey breeder; he's a complete pygophiliac.