Who is Scoutmaster Kevin?

I found few explanations for the scene. Couldn't find any official statement so I'm putting both for you.

One is it's a reference to Josh Brolin's 1980s cult classic movie Goonies. Deadpool have few lines from the same movie which made Brolin famous.

Well, as Scoutmaster Kevin used to say... "There's a first time for everything,son." Give me your best shot, One-Eyed Willy.

One-Eyed Willie was the infamous captain of the pirate ship featured in Goonies. It was Willie’s map Mikey found in his parent's attic, dragging Brand on an adventure he didn’t ask for. An article on thegamer had some references to the movie Basic Instict which Wade tries to do the same move like Sharon Stone (Catherine Tramell) did.

Another theory I found was that Deadpool hints about under-age abuse, with later revealing Firefist and more children were abused at the care-home they live. This scene is believed a hint to all the under-age abuse. As stated in theactionpixel,

the context and the uttering of two words alluded to sexual abuse of a minor. Talk of Fire Fist being abused also came up, which at first felt like it alluded to the sexual variety. Flashback scenes however just insinuated your everyday ol’ Abu Ghraib-esque Mutant torture sessions.

I'm not sure this''d be a good answer, but these are the things I can find for now. You can read the linked articles and find out some more.

It's a sexual predation reference.

It's difficult to not get explicit, but, wiping his face and then saying that is suggestive of it bringing to mind him having ejaculate on his face, and it evoking memories of his scoutmaster.

"One-eyed Willy" would be a euphemism for a penis - which is often referred to as someone's "Willy," and the "one-eyed" reference is also one that is often used in penis euphemisms because of the single opening at the end. It is not a reference to a pirate in a movie.

One eyed cave salamander One-eyed bath tub eel One-eyed blue-vein One-eyed cave dweller One-eyed Charlie and the Stink Twins One-eyed cornhusker One-eyed cow killer One-eyed custard chucker One-eyed Fred One-eyed giant One-eyed gopher One-eyed hip snake One-eyed horny newt One-eyed Jack One-eyed lizard with the saggy chin One-eyed milkman One-eyed moisture missle One-eyed monster One-eyed nightcrawler One-eyed snake One-eyed superhero One-eyed throbbing python of love One-eyed trouser mouse One-eyed trouser psychic One-eyed trouser python One-eyed trouser snake One-eyed trouser trout One-eyed wiggling welshman One-eyed Willy One-eyed wonder weasel One-eyed wonder worm One-eyed wrinkle-necked trouser trout

Dicktionary: The largest list of penis euphemisms

"There's a first time for everything" - referenced in another answer would be someone getting someone else to do something for the first time that they otherwise wouldn't.

Look at the image on that video capture - his head/face are at crotch level to the person in front of him.

Put it in the context of just about every other line in the movie - it's non-stop sexual innuendo and double-entendre.

It is, 100%, without a doubt, a joke about someone young being abused by being forced by an adult authority figure to give them oral sexual service.