-First. you made short circuit the IF output by connecting the PORT directly without decoupling capacitors.PORT element has 50 Ohm internal resistance and this makes DC voltage short circuited at the IF output.Connect a proper capacitor to decouple RF and DC..
-PORT element has already 50 Ohm internal resistance but you connected 50 Ohm in parallel that will make the PORTs resistance to 25 Ohm for nothing.It doesn't make sense, diconnect 50 Ohm parallel resistances at both RF and LO ports.