I know fanon asserts that the serum made sex better for Steve (either by improving his endurance, making him more sensitive, etc), but do you know of any fics where the opposite happened and maybe sex got harder for Steve but then Tony goes the extra mile to make it good for him? I’d also be curious if there were any fics like that for Tony, whether it be a symptom of extremis or something else?

I can only think of a couple like that, but here you go. Maybe others will have suggestions?

For Steve:

Super Soldier Serum by @tonystarkssnipples:
Not everything about the serum is a good thing

Salute by devlandiablo:
“After getting the serum, Steve thought he could no longer orgasm.
Whether masturbating, getting a friendly handjob, or sleeping with a
woman during the war, he just could not get there, leaving him
frustrated. He found some satisfaction from the build-up, but not
enough, and eventually just stopped even trying to have sex. Then, by
some happy accident or by letting his partner use his body, he discovers
that he can come through prostate stimulation, and suddenly he’s horny
as hell and eager to come over and over again.” Crossposted at LJ:
http://avengerkink.livejournal.com/7940.html?view=45368324#t45368324

For Tony:

There are some recs for Aging Tony that deal with impotence, so you might like those.  I don’t know of any extremis fics exactly like what you are asking, but there are a couple of “extremis messes things up for Tony fics” that are kind of what you are asking if you squint really hard. Not so much with it messing up the sex, but sort of unforeseen consequences kind of thing?  Anyway, here are some ideas and maybe followers have a few others.

No Consolation by valtyr:
Extremis lies to Tony. (Um…mind that tag!)

Virtual Realities (the Disappearance of Iron Man Remix) by antigrav_vector:
Tony takes Extremis. Things go sideways. Steve saves the day. It’s a Thursday.

Digital Wonderland (series) by tsunkinofaerii:
Post Civil War, without Reborn or the Skrulls. Tony finds other ways to try to keep himself together.

Frequency Modulation by TariCalmcacil:
After Tony receives the Extremis, the virus causes some unintended side
effects. Steve tries to help him deal with it and the changes it brings.

Astable by @ironlawyer:
Extremis was supposed to make him stronger, but after the fight with
Mallen, Tony loses control and Steve struggles to handle a Tony who’s
falling apart at the seams.

Our Love is a Ghost that The Other Can’t See by @kiyaar: Post-Civil War, canon-divergent from Director of SHIELD.
In Tony’s dreams, he is back in the street, in a crater, the city
on fire around him. Steve kneels over him and beats him to within an
inch of his life.

It’s his favorite place to be, these days.