The Voice of Inexperience: The Mathematics of Threesomes

“Dear BC,

When I’m having my nightly unprotected threesomes, something keeps nagging at the back of my head. Does having sex with two women at the same time double or square my odds of getting VD? Man, this has been bothering me for a while, but I just don’t have the background in probability you do. Can you crank this one out for me?

Sincerely,

E. Nonimous”

Dear E. Nonimous,

There may not be another person in entire world with both my mathematical background and perverted sense of humor. Therefore, I shall endeavor to answer your question not just in the two woman scenario but in every other scenario as well.

First let’s review the fundamentals of probability. Everything that can happen, has a probability of happening between 0 and 1. If it never ever happens, it has a probability of 0. If it happens in every case, it has a probability of 1.

Furthermore we know that the sum of all the probabilities of all cases must equal to 1, since something always happens when you set yourself to observing. Taking this even further, we know that the odds of something not happening are equal to 1 minus the odds of it happening.

If you can keep just these first few concepts in mind, you’re a long way toward understanding your problem.

Now, here’s where it may get a little tricky. We’re going to have to declare two terms. One of them is a variable (meaning it changes) and the other is some constant (which doesn’t change) we don’t know.

Let D represent the probability that a random woman has VD.

Let n represent the number of women you’re having sex with at one time.

To put it in simple terms, D is the constant you’re trying not to hit when your putting your manhood into some suspicious n.

So here’s the deal. We could try to calculate the odds that any one of the women you’re porking has something catching. Except that would get really ugly. In fact, the only case in which you won’t get VD is the case in which all of the women aren’t communicable carriers.

Remember, the odds of something not happening are the odds of it happening subtracted from 1. Now the odds of something happening twice in a row, like flipping two heads on a quarter is the probability of it happening once multiplied by the same probability. Therefore the odds of something happening n times, is the probability of it happening once multiplied by itself n times. So the probability of all the women you’re boning in a single night not having VD comes to….

All safe = (1-D)^n

Now that we’ve got this doozy taken care of here’s the final piece of the puzzle. The probability of you getting a venereal disease is the probability of you not getting a venereal disease subtracted from 1. Therefore…

Bad Bare Backing = 1 - (1-D)^n

From here on out, it’s just plug and chug.

Let’s take some guess work: the high end infection rate of chlymdia in a sexually active teenage girl is about 25%. Let’s hold this to be true for adults over say the age of 21, although in reality the number is probably much higher especially in women who would have a threesome. We now get.

Bad Bare Backing = 1 - 0.75^n

This means that if you’re just sleeping with one woman you’re odds of being infected are about 25%. Again, that’s a best possible scenario estimate.

Now, if we increase n to 2, in the case of a threesome, we find that your odds of infection are suddenly 7:16. With three women it’s even more. As you sleep with more and more women simultaneously, your probability of infection rapidly approaches 1.

So you see, while both scenarios you gave felt intuitively plausible, with just a little bit of probability we see that it’s just the tiniest bit more complex. Hope this helps you decide to use condoms.

Yours truly,
BC Woods

PS: This isn’t going to end people, I can keep making up questions or you can send some in.