The only circuмstances I can think of, where one might even remotely be able to justify it, is if a woman is married to a brute of a man, who insists upon an unjust fulfillment of the "marriage debt" --- let's say they already have six children, he spends up his paycheck on gambling and booze, and comes home drunk and libidinous (assuming both things can coexist) and insisting on sex --- and she is not able to resist without grave harm (or worse) coming to her and her family. In essence, she is being raped, and both the husband and his seed are unjust aggressors. I welcome responses, and in any event, I submit myself to the traditional teachings of the Church on this matter, assuming such a casuistry has ever been done.
Failing this, then both the wife and the husband have free will, and nobody ever has to have sex, otherwise, it's a species of rape, as I noted above. They don't have to, they just want to, but they don't want the consequences. IOW, they want to use each other to masturbate. (What else would you call it?)
And contraceptives fail. What then?
Nobody ever got pregnant by practicing abstinence.