Tony loves Steve and Bucky. He does. But he would maybe love them a little more if they would stop treating him like he is glass and will shatter if they are anything other than perfectly gentle and perfectly sweet and perfectly boring in bed.
A shopping trip with Natasha to a few (cough) adult stores... and Tony is well and truly armed with everything he needs to push his sweet, protective boyfriends over the edge into something a little less gentle and a lot more fun.
Tony is determined to get what he wants from Steve and Bucky-- leather and lace and handcuffs included
Unbeknownst to the general public, the serum gave Steve tentacles. Bucky never blinks an eye at them, but it does become an issue when they get together with Tony. Steve can't parse what the hell Tony is thinking when he stares at his tentacles, but it can't be anything good.
Steve is very wrong about that.
Or: Steve and Bucky have never heard of tentacle porn and Tony despairs at their ignorance.