but if i were to be short the dec 1240 put
straight out
and say a bomb would explode and the sp would trade at 600, my losses would be enormous
were as now, i am short the fut contract
what I'm trying to tell you is that MTE is correct. as he's made clear your trade is a complicated way of being short a sep call and short a dec put, a strangle spread between sep-dec. that is not a covered position. short a strangle = short vol. the calendar aspect just makes it slightly more complex.
think of it another way. you're long es and short es in another month. but you're short two options.