If it is not okay to use a person solely as a means to an end, then what do we do with other persons? We LOVE them! Love is the opposite of using. If "first comes love, then comes marriage, then comes the baby in the baby carriage", what comes before love?
The sharing of a common good or aim.
JPII writes in Love & Responsibility:
"Obviously, I may want another person to desire the same good which I myself desire. Obviously, the other must know this end of mine, recognize it as a good, and adopt it. If this happens, a special bond is established between me and this other person: the bond of a common good and of a common aim" (28).
Whenever two people share a common good, each of his own free choosing, then they see one other in relationship as equals. One is not "subordinated to the other," but rather both are "subordinated to that good which constitutes their common end" (29).
Unlike animals, love is "exclusively the portion of human persons;" love is predicated on free will, which animals do not posses as we do. Interestingly, just because someone shares a good end with another does not mean that they are truly able to love those others. It is possible to be utilitarian and to view another with a consumerist attitude while striving towards a good. Just because a man strives towards a good (i.e. protecting the unborn) does not mean he is willing to consciously pursue that good with others, subordinating himself to the good for the other's sake and subordinating himself to the others for the sake of the good.
JPII gives the example of employee/employer relations. It is clear that an employer can use his employee in a utilitarian fashion, to accomplish a company goal, but not really care about or get to know his employee as a human individual. They are both working towards a common good, but there is no love or common bond between them. (We are not yet speaking of romantic love here. Hopefully, that's obvious. ;-D) But it is also possible that both parties could be so persuaded of the goodness of their common aim, that they work together in a partnership, with an attitude akin to love, a camaraderie.
In marriage, it is possible that the man and woman acting together in their common sexual life, becoming one flesh (see Genesis 2:24), can use each other primarily as an object for sexual gratification. Married sex can still be selfish. JPII says the married couple should recognize that they share a common end. When it comes to married sex, the common goal is procreation, the creation of a family, as well as deepening the relationship between husband and wife. Dr. Janet Smith (Contraception: Why Not?) summarizes this point by saying that sex is for babies and bonding.
The couple value one another as persons, and value their future children as persons, and they are most likely to treat one another in accord with their dignity as human individuals. When it comes to sex, it's funny because people can actually mutually agree to use each other (i.e. porn actors, premarital sex). However, they are not supposed to. Also, it is possible that one spouse could be engaging in sexual intercourse with the other for the sole purpose of creating a child, in a utilitarian sense. (Not to mention that we cannot technically force conception. God controls life.) This would be also a selfish act, stripping love of the other person and the desire to grow in intimacy from the sexual union. Is this what was commonly practiced among Puritans in the early American colonies? Sex is for babies, but also for bonding.