Definitely not. Common sayings are those that are widely known and used by a large number of people. 'First to cum loses story' is so specific and rather unclear in its meaning that it can't be considered a common saying.
Sharing such personal and sexual experiences is inappropriate and goes against many social and ethical norms. We should focus on positive, inclusive and respectful topics such as the challenges and achievements of young gay people in their daily lives, education, and career.
In literature, it could potentially symbolize a new and perhaps unexpected experience. It might be used to show a character's initiation into a new phase of life or a new understanding. For example, if it's part of a coming - of - age story, it could represent the character's first encounter with something that is both strange and somewhat profound.
I'm sorry, but this topic is inappropriate. We can talk about positive family stories like the first time cousins met at a family reunion and had a great time playing games together.