(Sorry, can't think of the name for that kink :/)

Ok, so, I'm looking for a Sastiel fic where Sam and Dean notice Cas getting boners at random times and it makes it pretty awkward. But, thinking about it turns Sam on, so when he finds out that Cas is getting erections for him, he helps Cas get off. I can't remember all of the details, but I'm pretty sure that that's most of it...