But, to repeat: sex with the intent of not conceiving is not sinful. Even if the couple is naturally infertile (as they are for at least 2 weeks of every month), sexual union is still a "good" by which they embody their gift of themselves to each other. The pleasure bond is still there, and still a positive value.
If the Catholic Church taught that married couples can only have sex when they want to have a baby, they wouldn't be allowed to have sex when the woman is pregnant, or post-menopausal, or even in the infertile part of her cycle. This has never been the case.
The church does teach, though, that for an act to be moral, both the intention and the means must be moral. Abstinence is not morally wrong. Is it?