Guy friend initiated an MFM threesome, then promptly got forgotten once my pants came off
One NYE in my college years I attended a party at an apartment a few of my friends had together. I and a couple others wound up fairly drunk and a bit sleepy lying on my buddy's bed in the dark around midnight. It was me, my buddy who was very open about having a below-average size dick (bless his confidence) and our friend Jenny. The two of them started fooling around while I was just sorta half passed out on the bed, and eventually they started fucking somewhat softly and quietly. I could hear her contentedly sigh now and then. Eventually she hopped off of him and started sucking his dick with her ass in the air towards me, and they both half-jokingly encouraged me to join in.
Now I've never been one to brag in real life about sexual exploits or dick size or anything of the sort so at this point in time neither of them or anyone at that party for that matter had any idea of what I was working with. I decided to take her up on the offer and dropped my pants, though the room was lit only by moonlight so neither of them saw much or were even really looking. My cock flopped out heavy and rock hard, bobbing as I got in position behind her. I distinctly remember looking down at my cock throbbing inches from her pussy and just thinking "huh, she hasn't even seen my dick, and I'm a lot bigger than [buddy]..."
As I grabbed her waist and started entering her from behind, her entire demeanor changed on a dime. With her mouth still on his fun-size dick she let out a shocked and muffled moan as I stretched her wide and deep. I withdrew and started pushing into her again, and she pulled her mouth off his cock entirely, letting out a strained "OOOH FUUUCK" which she repeated in increased pitch synced with my thrusts as she buried her face in the bed sheets. I remember my buddy sitting there off to the side awkwardly, literally dick in hand, as Jenny's legs trembled with each thrust of mine and her moans turned to gasps and whimpers, her attention entirely focused on the unexpectedly large dick now repeatedly bottoming out inside her. Eventually he got the hint that he was entirely out of the picture for her and disappointedly got dressed to rejoin the party and leave us to our own activities. We wound up fucking on his bed, bedroom door locked, until about 2am before taking our private party down the street to my apartment and continuing until about 4 or 5. It wound up only being a one-off thing, but we both got to laugh about being stuck with with sore crotches (and pounding hangovers) for New Years Day and having one hell of a night to remember.
He was surprisingly cool with the whole thing, but he still likes to give me shit for essentially cockblocking him purely with my dick.