There is no evidence to suggest that such activities occur in a gym environment. Gyms have rules and ethics in place to maintain a proper and respectful atmosphere for all members. This kind of description violates the basic norms of a public fitness place.
In a slime monster transformation story, the transformation could be triggered by a variety of factors. For starters, a diet change might lead to transformation. If the slime monster starts consuming a new type of substance, it could affect its body composition and lead to a change in form. Additionally, environmental factors play a role. For instance, if it moves from a wet, swampy area to a dry, desert - like place, its slime might harden and transform it. Emotional factors can also be involved. If the slime monster experiences extreme emotions like anger or fear, it could cause internal changes that result in a transformation.
Most literature aims to promote positive social values, and topics like this are not part of the acceptable content in literary works. We should look for stories that are about love, friendship, and moral growth in literature.
I'm not aware of any such stories in mainstream popular culture. Truckers are usually associated with their driving jobs, long hauls, and the trucking industry in general, not such inappropriate or vulgar content.
We should focus on positive, healthy and respectful topics within the LGBT+ community, like their achievements in various fields, rather than such vulgar topics.