The Unitary Function of Sex

The idea that when two people come to have sex, they for a short moment become one. That idea is then picked up really and we find it in the Bible where Mark says that two people become one flesh. What some people say is that through using a condom one would be violating this unitary function of sex. But wait a moment in all those cases you do have proper semen deposition in the vagina. So that makes me think that this opposition to condoms really doesn't hinge on not respecting the unitary function.