I think I know what happened, Nick was desperately searching for something to throw back at Aaron that didn't incriminate himself in gay shenanigans and so he asked April for anything, April then either told the truth or made up a story about Aaron licking Nick's cum of her chest and Nick ran with it. (I feel queasy after writing that). Nick's probably of the mindset that if he was the top/the giver, then he isn't a fag, he's wrong, and that this makes Aaron look like the gay one, it makes them both look gay but Nick is the only one admitting to it.
The more he talks about this the more gay he seems, I mean this is the type of shit a straight man would attempt to repress and never think about again, not Nick though, Nick has to let everyone know that another man guzzled his jizz. Like a fucking badge of honor, like he's humble bragging or something.