Sex and Safety (The Mathematics of Condom Use—to be Geeky)

Let me preface this entry by saying I was raised Catholic. I attended Catholic schools and I attended Catholic Mass for most, if not all, of my formative years. It is the position of the Catholic Church that sex outside of marriage is a sin. I believed that all the way up until my first day at college. Then, I pathetically crumbled under peer pressure.

Anyway, I just want to point out a few things that somebody should.

Is there such a thing as safe sex? I would say, no. There is only safer sex.

Let’s deal with condoms. Last time I checked the stat, condoms had a 10% failure rate. Or, a 90% success rate. Which seems like a good thing and an excellent precaution to take before having sex. However, let’s not kid ourselves: Alone, condoms hardly provide a silver bullet of protection. A 10% failure rate means that condoms will fail once every ten times they are used. The problem is that sex is not something you do just once and never again. The sexually active have sex repeatedly.

Assuming you are using condoms in your relationship, a reasonable estimate of the frequency of sex is probably about three times per week. If we allow one extra act of copulation over a three week period that means after three weeks a couple will have had sex 10 times and, with the failure rate of 1 in 10, that means the couple will have their first statistically guaranteed condom failure within that time period.  Now, how dangerous is this?

The average menstrual cycle is about 28 days. We’ll be generous and say 30 days. Now, a woman is—I’ve forgotten the correct word—capable of being impregnated for a (we’ll say) 3 day period over these thirty days. So, if a woman has sex on a random day, there is a 1 in 10 chance that she will be impregnated. So, if there is one condom failure every three weeks resulting in a single random impregnation attempt, there is a 1 in 10 chance that the woman will be impregnated after three weeks of a sexually active relationship. The math here is pretty simple: from here, it is apparent that after 30 weeks, the condom will fail 10 times. As the chance of impregnation is 1 in 10, the chance the woman is impregnated after 30 week is 10 in 10, or 100%.

Let me repeat that: if a couple only uses condoms and they have sex three times a week every week, by the time week thirty rolls around, the woman is statistically guaranteed to become pregnant. It’s possible my stats are off (I never took statistics, but this is pretty basic math … all 1 in 10’s and stuff), but I’ve gone over this a couple times. My math appears accurate to me.

That’s pregnancy. Thirty weeks (or approximately seven months).

As for disease, it all depends on the degree to which an exposure occurs with every failure. Perhaps a failed condom usage still provides better protection than no condom at all (for that one time—obviously, for the nine times it works, it is better protection). I don’t know. What I do know is that after three weeks there will be some, at least limited, exposure … statistically speaking.

So what does all this mean? Particularly with how we educate the young regarding sex?

I think an ABC approach is probably the best. Actually, I just looked it up: ABC is Abstinence, Be Faithful, and use a Condom. I originally thought the ‘B’ meant Birth Control, not Be Faithful. Anyhow, given the above, condoms alone aren’t good enough. You should probably throw in other birth control measures like the pill (I think they are actually assumed in the ABC approach, but I wanted stipulate that with certainty). Of course, some people have moral problems with that (as well as condoms, or even sex in general), but I’m not going to explore that topic here. I was only interested in discussing the “mathematics of condom usage.”

Somebody should. It’s not safe sex; it’s safer sex. And that is an important distinction to make, because over time your odds get progressively worse. Seven months. That’s how much time you buy.


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