This one's actually inspired by a line in Avengers Assemble, when Stark says to Loki "Performance issues, not uncommon. One in five guys, so I'm told". And then this happened. But it fits with the PTSD theme of IM3.
Performance issues not uncommon. He was an engineer, he knew that. He knew that without proper maintenance, problems would occur.
It wasn't just one thing. It was lots of things, all related and yet totally different. The nightmares, the panic attacks, and this. The lack of... well. Not uncommon. One in five, so he's told.
"It happens to a lot of guys, Tony. And you're not as young as you used to be."
He doesn't tell her how long this has been a problem. About how he hasn't been able to... you know... with another person since Afghanistan. When he's alone, sure. In the shower, he'll jerk off. He'll wake up with morning wood sometimes if he sleeps by himself. But when other people are around he just... can't.
He and Pepper weren't together and then he was dying and then they were together but they were waiting and so she doesn't realise. She doesn't know that his own body is betraying him and he can't decide if he prefers it when he can get an erection or when he can't.
It's about trust. He knows that. Same as half his other problems. Trust and fear. He'd been in a cave for three months. That's a long time. He'd been tortured, obviously. And there had been Yensin. Poor, dead Yensin.
And when he came back his heart was broken and his dick didn't work anymore.
He'd told himself it was his injuries. To give himself time. Then it was the alcohol that was doing it. or the palladium poisoning. But all of those were gone now. He'd run out of excuses. He just couldn't. And he couldn't explain it to Pepper while she was so patient and sympathetic.
So he goes down on her instead. Not exactly a hardship. And he thinks about those little blue pills.
One in five. Not uncommon. He just had to keep putting on a good show and no one would know how broken he was inside.