The probability of indirect human fertilization (follow-up)
Safer sex techniques are more risky.  Perfect condom use is associated with around a 2% pregnancy rate within a year.  According to that, even sterilization has a 0.1% (male) or 0.5% (female) rate.  Combining techniques is, of course, safer.  "Indirect," as you proposed in the other question, will border on an infinitesimally small chance, as answered there.