The OED definition of taboo is as follows:

Set apart for or consecrated to a special use or purpose; restricted to the use of a god, a king, priests, or chiefs, while forbidden to general use; prohibited to a particular class (esp. to women), or to a particular person or persons; inviolable, sacred; forbidden, unlawful

I am not sure how sex can in any way, shape, or form, be considered "taboo" in Christianity, except for perhaps the clerical class of some Christian sects.