This guy (PurpleMonkeyDishwash) does a lot of stories about it, but it tends to be tied up in other genres too, this is probably the most straightforward.
Double Date - Fetish - Literotica.com
Also basically this guy's whole deal, though it focuses more on everyone being pure and upright (there's usually a religious leader) until exposed to the big one.
Literotica.com - Members - NTRmaster - Submissions
He's got a website with more
Unrelenting Cruelty
This guy, too, has a whole series about it. It focuses on a father who enjoys stealing the girlfriends of his son.
Literotica.com - Members - acdd123 - Submissions
This is a fun trilogy
The Allotment - Loving Wives - Literotica.com
And for people who want a complete reverse of the standard and some of the annoyance a woman might have in the situation.
https://www.asstr.org/files/Authors/she_cries/Dara and Kyle.txt
An interesting one from a great writer
Ashley's Corruption - NonConsent/Reluctance - Literotica.com
Stormbringer has, of course, dabbled in this genre, as he has others. An interesting one, in this case it the other guy is very sneaky about it.
STORIES BY STORMBRINGER: The Bigger, The Better
And there's this
Amanda, My Sweetheart - Exhibitionist & Voyeur - Literotica.com
And this one, which cuts some to the quick but involves a slow reluctance being won over, not once, but twice. Probably my favorite.
The Good Guy Always Loses Ch. 01 - NonConsent/Reluctance - Literotica.com
Careful on this site, it's full of bad half-assed stories and hate. This is an old one, from before the site changed a bit. In this one it's not -just- size, but also experience that really wins her over. Though the size is still there.
. - Anything For a Friend
I'm checking out the stories everyone listed above, thanks Moose99 for keeping this active. So many stories and interests get lost to time. I've also read fantasyfunchat, though I preferred the slow corruption of "Former Good Girl, Now Gangbang Slut."