The thing is, when i look at this pic, i see a swimmer, you see semen:
Just the same with bodybuilders in posing trunks. You see sex, bodybuilders see contenders.
Swimmers do something different compared to bodybuilders. Even if swimmers have visually pleasing bodies and one that is widely deemed ideal, the appearance of their bodies is not the center of attention nor does swimming revolves around the idea of appearance. They prepare their bodies for maximun exertion of its capacity to swim as fast as possible.
Bodybuilding, on the other hand, is about appearance, they are required to look a certain way and when compared with other contestants, the one whose body meet the criteria (under a very subjective opinion) will get the prize.
Whether or not I see sex I think you simply cannot deny the fact that sex, as you put it, is for most bodybuilders one of the motivations to bodybuild. Whether or not I see sex it is a fact that most bodybuilders don't reject the idea of being perceived as sexually more appealing than the rest of the male population, let alone the fact that the whole bodybuilding world is about exploiting the sex appeal of muscles both implicitly and explicitly. I haven't heard one bodybuilder wanting to change this about bodybuilding; instead, most pursue the possibility of appearing in mag covers of being photographed for different purposes as the ideal of male prototype. I didn't even mention what people call G4P because the use of sex in the bodybuilding industry is much broader than G4P.
Please, tell me how puritan is this.

Is there anything wrong with it both from the perspective of the bodybuilder and the spectator? Not at all. Putting your nearly naked body on a stage and one that is glorified because of its enhanced secodary sex traits conveys an assumed idea that people can see it from a non-sexual and sexual perspective or mix of the two at the same time. Whether or not people watch bodybuilding because it is sexually stimulating to the eye no one gets to morally judge that. Do you not watch female competitions because their bodies please your eyes?