How about someone looks it up.
ath·lete /ˈćθlit/ Pronunciation Key - Spelled Pronunciation[ath-leet]
–noun
a person trained or gifted in exercises or contests involving physical agility, stamina, or strength; a participant in a sport, exercise, or game requiring physical skill.
Bodybuilding doesn't require physical agility, or stamina, or strength actually.
So could it be a sport? Well, let's look that one up too.
sport /spɔrt, spoʊrt/ Pronunciation Key - Spelled Pronunciation[spawrt, spohrt]
–noun
1. an athletic activity requiring skill or physical prowess and often of a competitive nature, as racing, baseball, tennis, golf, bowling, wrestling, boxing, hunting, fishing, etc.
Hmmmm, well, bodybuilding is competative, buuuut then again, that's not obligatory to be a sport now is it. So, does bodybuilding take physical prowess then? To the dictionary one last time...
prow·ess /ˈpraʊɪs/ Pronunciation Key - Spelled Pronunciation[prou-is]
–noun
1. exceptional valor, bravery, or ability, esp. in combat or battle.
Does a bodybuilder need valor? No. Bravery? No. Ability? Well, how do you feel like defining ability. If that includes genetics yes, but the fair majority of bodybuilders are working with crap for genetics, so nope. And Flex Wheeler is the only person to ever refer to bodybuilding as combat or battle.
Then, by definition, bodybuilding is not a sport, and bodybuilders are not athletes.