Neither would I. I think it likely that some of the first uncrewed cargo Starships going to Mars (before any humans make the trip) will contain ISRU technology that will be designed to operate autonomously to test the procedure. If it works, then when the first humans leave for Mars cargo Starships will also leave at the same time with the ISRU equipment.I wouldn't send a crew to Mars without the propellants already in place, but that's me.
As to exactly how the O2 and CH4 produced on Mars will be moved from the ships containing the ISRU equipment to the ships that will be making the trip back to Earth I do not know. Maybe they will land ships within 100m of each other and use hoses and pumps? The QD ports are far above ground level so that will require some lifts.