Instead of jumping to conclusions about why Catholic couples use contraception (as you did), I am suggesting we ask them and let them answer.
1) They don't think it's really sinful, because they really don't believe much is sinful nor do they have an understanding of the nature of sin. Many of them were never really taught about sin, just luv, luv, luv.
2) They believe God is all loving, and just wants them to be happy therefor he could not possibly ask them to do something that might require such a personal sacrifice.
3)They think that being a good Christianity means being a nice person and good neighbor.
4)They don't believe in hell, and they certainly don't believe that they could ever be punished eternally, after all, God is love.
5) And almost every one of them has had a priest or deacon tell them that it is perfectly OK, as long as they are following their conscience.
"Instead of jumping to conclusions about why Catholic couples use contraception (as you did), I am suggesting we ask them and let them answer."
You mean we don't already know? They want to enjoy sex anytime they like, but not have to worry about a baby. Following NFP entails work and, at certain times, abstinence.