The England wing insisted he had touched the ball down one-handed as he was tackled and flipped onto his back by Leigh Halfpenny, George North and Jonathan Davies.
With time up on the clock, a try would have given Toby Flood the opportunity to draw the game for England with a touchline conversion.
Referee Steve Walsh sent the decision upstairs to television match official Iain Ramage, who eventually ruled it was not a try because the footage was inconclusive.
Frustrated Strettle thought touch-judge Pascal Gauzere should have offered referee Walsh more assistance as he was standing right next to the incident.
And once the decision went against England, Strettle insisted referee Walsh should have brought play back for the penalty because he was playing advantage.
"I thought I had scored. I thought when they flipped me on my back I had got downward pressure. I felt the ball touch the floor," Strettle said.
"When they showed it on the big screen it looked like it went down as well.
"The strange thing for me was that I was hoping the touch judge might have seen it because our physio was standing right next to him and he says it was grounded.
"If it is inconclusive you have to go with the attacking team. I thought we had the penalty as well. I can't understand why we have not gone back for that penalty."
Strettle also questioned why North had not been penalised a few minutes earlier for deliberately knocking the ball into touch as the two wings chased Flood's cross kick.
"There were a few decisions - when Floody kicked the ball through and it was knocked into touch (by North), you can't do that. He wasn't making an attempt to take possession," Strettle said.
"Obviously it is very disappointing to come off without a win, especially to have come as close as that try when we could have drawn the game.
"It is frustrating but we have lots of positives to take from the game. It is how the team responds now to our first defeat.
"We showed the public what we are about as a team - we are good in defence but we can take it to teams in attack as well."
The decisive moment came with the scores tied at 12-12 when Wales' replacement centre Scott Williams ripped the ball off Courtney Lawes and regathered his own kick ahead to score.
England managed only four penalties from fly-half Owen Farrell but produced their best attacking game of the RBS 6 Nations so far, with Manu Tuilagi making an eye-catching return to the side.
"As a whole we have created a lot of opportunities and attacked well," Strettle said.
"We have been frustrated the last two weeks (against Scotland and Italy) that we have not been able to show the public we can go and play rugby and we want to go and play rugby.
"I think they came down to physically dominate us but we stood toe-to-toe and smashed them back."
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