That is strange because the green wire to the PSU is power control input. The power switch applies a ground to that wire. The motherboard simply passes the signal through to the PSU. If the PSU is working properly properly, you will see 5 volts on that line. When you press the power switch, that voltage drops to 0 volts. This tells the power supply to turn on. If you press and hold the switch for about 3 - 5 seconds, that tells the PSU to shutdown.
When you press the switch, only a small current flows (.002 - .005 amps).
My guess is that you have something wrong with your power supply.