Leighton Baines was bundled over by Cardiff midfielder Gary Medel in the first-half inside the hosts' penalty area.
Martinez said: "From the angle I was at it looked a real clear, stonewall penalty.
"Leighton said it was a clear penalty. He was trying to get into the box and there was a challenge where he doesn't get the ball. It's as clear a penalty you will ever see.
"The last thing I would do is come out and just blame the referee for not getting the three points, but it was a key moment. The first goal would have been significant in this type of game."
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