I *like* erotic stories, and I like stuff where the sex is only implied, also. I mostly read first time sex stories. It's fine if the first-time sex blooms into a long-term relationship. However, I've not found many stories I've enjoyed that start with two older out gay men. I don't know why -- perhaps there's not as much drama in that.
For any story, I want to know why they decided to climb into bed together. I can't stand it when a story _starts_ with two guys having sex. The intro doesn't have to be long, but it should tell me why I care about these two guys doing "it".
First time sex doesn't have to be between twelve-year-olds. And that's probably not realistic anyhow. I have a "friend" who was 28 before he decided/admitted he was gay, and it was another six months after that before he acted on it. And no, he didn't turn into a slut immediately. It generally takes awhile to build up to full slut status.
The best stories have at least a hint of truth to them, particularly about the sex. Throwing in something truthful (like how your butt gets covered with sand when you try to have sex on the beach) helps make the wild orgy scenes more believable. Write what you know. There's no need to describe every thrust -- I have a *vivid* imagination.