So long as your marital relations remain open to having children should God bless you with them, you are fulfilling the requirement to be open to life.
That said, I think you and your fiancee need to reconsider whether you are being called to marriage. One of the purposes of marriage is for the procreation of children, to raise saints for the kingdom of heaven. Not every couple is blessed with biological children, but for a couple to deliberately seek to completely and totally avoid ever having children is troubling, even when the means used to do so are otherwise unobjectionable.
Marriage is a vocation that requires the sacrifice of self and one’s own desires for the good of the other. If you close yourself off to self-sacrifice on behalf of children, it is a very short step to refusing to sacrifice oneself for your spouse. And although children cannot and should not be expected to save a troubled marriage, their presence can mean that you and your spouse will work harder to make your marriage work. Without a young, innocent, vulnerable person whose interests are put above the interests of self-sufficient adults, the motivation a couple has to solve marital problems is severely weakened.
In short, if you don’t ever want children, you really should reconsider the vocation of marriage altogether. It is very possible that neither you nor your fiancee are truly called to it.
