The Roman Catholic Church's position on sex is very logical, if a person accepts their first assumption. The first assumption being that there is a reasoning creator behind the universe.
Another thing that has to be assumed by the church is that humans should not have control over their reproduction. Indeed that humans should not do anything, other than avoiding sex at the time of the month (the rhythm method), to prevent conception. No condoms, no pills, and especially no abortions. (The last one being the only one I am even a little sympathetic with, not that it is terribly relavant.)
Though there are people that believe implicitly in what the church teaches many members recognize that the doctrine of the Church was arrived at by accident or prejudice and then sustained through tortured logic to preserve the perceived legitimacy of the institution. For example one of the primary reasons sex is not allowed to the priests of the Latin Rite of the HRCC is that early monks viewed sex as unclean. When a monk became pope in the 8th century this was imposed as a rule for the whole of the church. Likewise with condoms. It was once thought that semen contained little fetuses and so using a condom (or masturbating) would be in effect infanticide. Now we know better, but the position of the church has not changed to reflect this new knowledge. They have come up with new reasons for the old proscriptions, but this does not make them any more logical.
When the question of what a person or a church believes in I think it comes down to a matter of faith. Either a person believes and the explanations help to make him feel that he is right or he does not believe and finds the reasoning utterly unconvincing.
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