If Tony was stuck on the other side of the portal, he'd be dead
Tony was dying in Avengers 1, because his suit was not capable of space travel. So the idea that he was able to survive not only the vacuum of space, but also the blast of the nuclear weapon he carried into the portal, is just dumb.
***
Tony KNEW he should be heading back. The bomb was about to unleash a can of nuclear whoop-ass on whoever sent the Chitauri and Loki to Earth. His suit also couldn't handle the lack of atmosphere and pressure much longer.
But still he lingered. Looking at everything.
He lingered too long. The warhead went off.
Everything flashed white. Then everything went black.
***
Pepper, Rhodey, Happy, and the remaining Avengers were gathered at the device Erik Selvig had built.
Bruce said, "We had to do it. The fallout would have come right through the portal, maybe even the blast itself."
The others nodded.
Steve asked, "There's no chance he could survive?"
There was the sound of a throat clearing from Pepper's phone. Jarvis said, "I have run all of the calculations against the specifications of the armor Sir was wearing. Even if there were no explosion, he would not have lasted longer than another minute in the vacuum of space."
The others looked grim.
Rhodey said, "At least Tony Stark died a hero."
***
Ironically, Tony did a better job protecting the universe by dying in Avengers 1 than he did staying alive to cause harm for another 11 years.