Using Viagra does not in any way inhibit (contra-) conception, in fact enhances it, and does not therefore in itself separate the unitive and procreative aspects of sexual union.
How’s that?
Exactly. I strongly disagree with those who say Viagra frustrates the procreative purpose of marriage. I think that the logic in their argument runs as follows (correct me if I’m wrong):
Since a couple using Viagra might suddently get the urge to have marital relations with the primary motivator being desire to be intimate with one another and not procreation, then the unitive and procreative aspects are separated. But if you apply this criteria for the sanctity of marital relations to the entire history of sexual intercourse between spouses, before contraception and NFP, then its a fair bet that almost every time a married couple wanted to be intimate with one another, no matter how open to life the intercourse was (lack of any means of birth control, natural or artificial), the intercourse was a mortal sin. Obviously, this position is absurd.
Objectively, procreation is the primary purpose for spouses to have relations. So, artificial birth control cannot by any means be introduced in the process. I don’t see how this translates to mean that, subjectively, each time a couple wants to engage in intercourse, their first feelings about it must be desire for children over desire to be with one another. These are feelings; they are beyond the control of the individual spouses.
Moreover, it would seem only natural, given our human nature, that desire to be intimate most often be the primary motivator for intercourse. A husband or wife is right in front of you, you know that you love that person and want to consumate your love with them, a child at that point is still a possibility that may or may not be realized 9 months later. Desire for intimacy more than children, by itself, in no way reduces the openess to life of sexual intercourse, especially since the couple may very well desire that procreation occur from the relations.
It would seem that Viagra only aids the process of procreation- instead of not having sex, a couple is having sex. I’d say there’s a greater chance of a woman’s pregnancy with the latter.