How to unplug this connector? Tried everything that came to mind.

 I found this photo online but I don't understand how I could do what lightning suggested because the flat screwdriver would press on the red gasket on one side. 

If you look at your picture, its already starting to separate. Look at the top and you can see the inner part starting to move upwards. Look at my video and you can just see this part move up at the early bit of the separation. Have you tried pushing it back together and then using the screwdriver to twist it out again, keeping it moveing in and out.

 Thanks guys, I finally managed to unplug it using lightning's method. The catch is to find a flat screwdriver of the right size and thickness and insert it between the gasket and the top of the male side of the plug, then make a lever.  Mine was really stuck, there was a loud pop when it went loose.
 Sadly the fuel pump is dead, so I'll make a new topic about cutting the floor to make a suitable hole for pump replacement.