I am not necessarily one who thinks marriage is the natural vocation and therefore if religious life is ruled out that means marriage -- because I think the single life without being called to the religious life is also a vocation. So I think there is discernment necessary for all. I have never doubted my vocation.
I think it is the same with children. Marriage has two purposed the procreative and the unitive decoupling the two creates something less than what it was supposed to be. However, just because spouses are called to an openness to life does not guarantee that they will be parents. Children are a gift of marriage but are not guaranteed. Likewise, openness to life is being open to the possibility of natural biological children as well as adopting children of all ages, or fostering etc.
So while a person maybe called to marriage they may not encounter their spouse. Likewise a person may be neither called to marriage nor to religious life, or they could be called to the religious life. People who believe their vocation includes children may find they are unable to bear natural biological children, but very well may be called to parent thirteen year old orphans or an infant from China or their sister's children, etc.
Biologically speaking humans are reproductive and social creatures who create reproductive pair bonds - making a pair bond (marriage) and procreating (having biological children) is within the Natural Law. In that sense it is a natural vocation.
Those who are vehemently opposed to children are not open to the gift of life in marriage and by that very conviction are unable to meet one of the requirements for a sacramental marriage -- the openness to life.
So as I see it there are three main vocations: single, married and religious -- a person may move through all three throughout their lifetime and some permutations of the three. As for children: there is the openness to life but no guarantee that a biological child will be the fruit of a marriage -- rather that openness may find its expression in biological children, step children, adopted children, fostered children, etc.