Do you mean sex?
If so, the repetition is not just repetition, is it? It's continued stimulation, through repetitive gestures, yes, but with a linear progression of emotion/intensity, that may go up and down, but is not exactly repetitive. In fact, the male is well known for his limited capacity to repeat the act.
I did mean sex, but I was just thinking, agreeing really, how a basic repetitive action fails to automatically equal
boring. A classic game example from the 8-bit days in the UK was "Daley Thompson's Decathalon" (aka "DT's Joystick Knackerer"), in which a nation of kids would happily waggle their joysticks to destruction... so to speak.
Off the subject of sex (and other bedroom athletics), I remember hearing in various documentaries that some agitated autistic children have a very strong calming response to repetitive motion, such as being swung back and forth by their parents in a bedsheet like a hammock. So they seem to be not bored but gaining some satisfaction from repetition too.