This is not about cheating, cucking, alphas and betas, "putting someone in their place" or homewrecking.

  You were the cutest couple one could think of.
Always have been, from the first time the both of you were making out at college. Cheered by everyone around you because you clearly made for each other.
And indeed you were. You never had a serious fight during the past six years Sex was stellar. He always puts in that extra bit of effort and never failed to satisfy you. Still, lately you felt like there was something missing. Maybe it was time to have a baby? Maybe, but not before both of you finished university. After all, you had a bright future lying in front of you. As individuals, as a couple and as a family. So why not go through two semesters of missing something. Something you didn't even know what it was.   That's when you took notice of her for the first time.
You've seen er many times before, the loner on campus that never attracted anybody's attention. Still you find yourself eyeing her from head to toe. Black hair in a pixie cut, worn out t-shirt, nondescript skirt or cargo pants, sneakers. As lift your head you are looking right in pale blue eyes.
And blush. And smile your most embarrassed "hand in the cookie jar" smile.

  The next morning you are locking your bicycle as a tap on your shoulder makes you jolt around: "Hi! I'm Charly!"
From under that lid lid of black hair a pair of sparkling eyes and pale pink lips shower you with empathy and joy.
And your mind starts racing.
Maybe that's what's missing. Spice things up a little. After all, neither you nor your boyfriend had sex with someone else. Or maybe just test him, will he be faithful? Make out with her in front of him to tease him a little? Why not try something new?! She wouldn't be a threat to your relationship.
Her flat chest, her cute but equally flat arse, that hairstyle and the chipped off nailpolish, the narrow but pronounced nose on her pale face, her wiry arms dangling from bony shoulders, all kept in balance by a pair of giraffe legs. The exact opposite of your features, the features your boyfriend adores so much.
So why not. You love him and he loves you, at least that's certain.
You just stare at her, your stomach churning while you hope she doesn't read your thoughts.
 

  Now I can't stress this enough: while she might be a little dominant or cruel at times, she's not a homewrecker. She just enjoys fooling around not discriminating any gender.
I want to explore your motives for Charly fucking him as well as the effects on your relationship and your boyfriend's life:

Maybe you want to give him a taste of his own medicine? Maybe thinking of him being fucked just turns you on? Maybe it's just a normal progression and you never planned this to go this far? Maybe you feel guilty because although you enjoy pegging him you just can't provide him with the "real thing"?
Does it tighten the bonds between you and your boyfriend, will there be jealousy or a notion of a "shared secret"?
Will he come out of this stronger? Or as a weakling? Will it make it easier for him to sleep his way up the career ladder and thus make your future even brighter?
Feel free to add other aspects.
 

Obviously, this would be longer term.
I usually write first person present tense.
Since I am not really bursting with creativity I respond relatively slow, writing one to a dozen paragraphs per reply/day with the occasional quicker back and forth strewn in. So I don't mind if you are slow and creative. At. All.
Also, I'm in CET, so probably way off your time zone.
Reddit PM for the time being, not that chat thingy.

Hard limits: scat (including a2m), blood, beast, vomit and gagging, non-con, heavy pain and degradation, male(!) feet. Generally all things misogyny as well as misandry (hey, when I rape you it's because you're a feckin' cunt, not because of your gender, right?!). All things hyper/growth/breeding. Kink List, FWIW.