Catholics don't view the body as evil -- they view the body (and thus sex) as good. As for science, it doesn't make moral pronouncements, so it can't view the body as good or evil -- those are moral terms, not scientific terms.
Just take a look at the Catechism: "2352 By masturbation is to be understood the deliberate stimulation of the genital organs in order to derive sexual pleasure. "Both the Magisterium of the Church, in the course of a constant tradition, and the moral sense of the faithful have been in no doubt and have firmly maintained that masturbation is an intrinsically and gravely disordered action."137 "The deliberate use of the sexual faculty, for whatever reason, outside of marriage is essentially contrary to its purpose." For here sexual pleasure is sought outside of "the sexual relationship which is demanded by the moral order and in which the total meaning of mutual self-giving and human procreation in the context of true love is achieved."138 "
Source: http://www.vatican.va/archive/catechism/p3s2c2a6.htm
Catholics see sex as a mutual gift of self between spouses. Through the language of their bodies, the husband tells the wife "I am yours," the wife tells the husband "I am yours."
As a gift, it is necessarily permanent. It's more than impolite to give a gift and then ask for it back. It's also exclusive -- you don't give someone a gift and then take it back and give it someone else -- the original recipient would rightly be aggrieved at such a turn of events ("Hey, you said that was mine!")
While all of marriage subsumes this gift of self, sex forms the act -- the sign -- signifying the gift. Sex is a physical expression, in the concrete language of the body, of the mutual self-gift that the husband and wife made to each other in marriage. The human reproductive organism consists of two persons: a man and a woman. Their union thus not only signifies their mutual gift of self to each other, but also serves as a creative act, a procreative act that generates life. Thus sex is both unitive and procreative, or in vernacular alliterative, both babies and bonding.
All that leads the Church to say that "the deliberate use of the sexual faculty, for whatever reason, outside of marriage is essentially contrary to its purpose."