How is having sexual intercourse during a non-fertile part of woman's cycle a "frustration"? If you want to call a portion of a woman's cycle a "frustration" go ahead but it doesn't hold up to reality. Chances are extremely low of procreating but not impossible. No matter how not-open to life a couple might be their intentions cannot alter the fact that they are doing nothing which deliberately stops a sperm from reaching an egg and creating new life.

I'll admit that on the surface the differences are subtle but the differences are also quite real. Again, personal intention while important does NOT determine what a moral act is. What actually happens determines what a moral act is. Dropping atomic bombs on cities is morally repugnant no matter our pure intentions to "end the war". Using artificial contraception is an act against an end of sexual intercourse. Having sex during low fertility portions of a woman's cycle is simply having sex. Full stop.