The pair clashed approaching Turn 4 on the opening lap, as race leader Rosberg - who had started in the wrong engine mode and was around 180bhp down on power - moved to the inside line to cover off polesitter Hamilton's attempt to repass him.
"He [Rosberg] wasn't on the racing line so I went to the right - there was a gap and I went for it," Hamilton said.
"When you're 17km/h faster and there's a gap, you go for it. That's what racing drivers do.
"I had part of my car - my wing and wheel - alongside, but then had to take to the grass. We saw what happened after that. I did what I could to avoid an incident, but it all happened pretty quickly."
Toto Wolff, head of Mercedes-Benz Motorsport, billed the contact as a racing incident - as did the stewards - and said it was impossible to completely avoid, given they allow their drivers to race each other.
"I don't want to start blaming one or the other," he commented. "Both Lewis and Nico are upset and we talked with them, looking at the pictures and the data, to determine exactly what happened. By letting the drivers race as we do, this kind of eventuality can happen but we won't change our approach - we owe it to Formula One and the fans to let them race.
"We have matured as a team over the past years, so we will be able to move on from this and, hopefully, fight back in a positive way in Monaco."