The Problem With Condom Use in a Marital Relationship

In Catholic theology it's permissible to use condoms in a marital relationship where one person is HIV positive and the other isn't. In Aristotelian way of thinking about morality why would condom use be impermissible? The idea is that we think about sex as instrumental for two people to become one, so as Marx has in the Bible for two to become one flesh. To engage in sex properly is to respect both of those functions.