You are right in saying that the Catholic position is that it must be open to life, but your unerstanding of what the Church means by ‘open to life’ is incorrect. The Church isn’t saying that what is necessary for being open to life is that conception is possible, if that was all then they really would be hypocritical to say that artificial conraceptionis sinful while NFP (or sex while pregnant, post menopausal, after one spouse has been sterilized etc) is fine. A lot of people think the Church is being hypocritical precisely because they do not understand (or refuse to accept) what it actually is that the Church means by this word. So if you are going to understand what the Church
actually teaches, the first step to that is making sure that you ignore the false understanding of the term ‘open to life’ which you have come to. This is not what the phrase ‘open to life’ means. Essentially, what is meant by open to life is that the couple must not do anything to mess with their natural fertility, ie use artificial means to render themselves, or a particular sexual act, infertile. The fact that they are currently infertile is not a problem, what is considered sinful is messing with their fertility in an attempt to artificially remove it from their particular act of sex. In which case its pretty clear that there is no contradiction in the Church saying that having sex while pregnant is not sinful. The apparant contradiction comes in through misinformation about what the Church means by ‘open to life’.