Here's how the order of things would work if we weren't fallen human beings. I'll use the food example.
IDEAL: I will eat this delicious meal BECAUSE, being delicious, it will bring some comfort to me, and this comfort will allow me to pray more easily, and this comfort of my lower nature will make me less cranky and less prone to sins of anger, and will allow me to more readily inclined to do God's Will (say, by giving me the energy to perform my duties of state better).
IMPERFECTION: I will eat this delicious meal BECAUSE it's delicious and it brings me comfort (with no thought for the higher ends).
VENIAL SIN OF GLUTTONY: I will stuff cake into my mouth because it brings me pleasure even if it leads to long-term discomfort and makes me crash so that I can't perform my duties of state.
MORE SERIOUS VENIAL SIN OF GLUTTONY: I will eat as much as I want because of the pleasure and will then go vomit up the food so I don't get fat.
Similar distinctions can be drawn for venereal pleasure. In the last step, with regard to venereal pleasure, it would be a mortal sin (it would be to prevent procreation during the activity). What you're doing there is to completely frustrate the final purpose or higher end of the pleasure itself.