Condoms really do not have a 2% failure rate everytime you use them. It’s 2% over a year. So if the average women in the surveyed was having sex once a week then it would be around 50 times less likely to happen in one er, usage. So more like 2/ 100x50 or a 1/2500 chance of falling pregnant from that one off. Of course there are things that we could take into account to make that stat more accurate. Like what point in her cycle she had sex with condom-wearing man. The odds of her getting pregnant from unprotected sex are more like 80-95% over a year (depending on age) and depending on what point in her cycle the sex happened, you could be looking at odds more like 1/3 for a one off occasion.
But this is not quite right either. I know that you said that it matters where she was in her cycle, but it matters much, much, much more than you seem to be suggesting. It is not a 1/2500 chance per occurence of sexual intercourse because that is not how conception chance works. This is a huge misconception that many people have, myself included until I started to look into it due to infertility.
Basically, a woman can only get pregnant if she has sex during her fertile window. It does seem that some women can ovulate more than once so may have multiple fertile windows, though I really don't know how common that is. But in any case, although you would logically think that having sex 10 times during the fertile window would result in 10x higher chance of pregnancy than once, that's not actually the case at all. The reality is if you have sex even once during your fertile window, there is around an 80% chance that at least one sperm will come into contact with that egg. So it doesn't matter if you have sex multiple times or once, because you're already going to have a zygote get created more often than not. You'd have a slightly higher chance if having sex multiple times, but the main benefit (if TTC) or risk (if trying to prevent pregnancy) of having sex multiple times is that most methods we have to determine the fertile window are not particularly accurate and/or only work in retrospect, so if you only have sex once, you might miss the fertile window all together. Some TTC methods even refer to shagging every other day as "carpet bombing" which refers to the attempt to ensure that the fertile window is hit.
Now I'm sure you're going to say but wait, it's not an 80% chance of pregnancy after one fertile cycle, and you're right, because what happens next depends on the quality of the sperm and egg. Even in a young, healthy couple with no genetic issues or sperm issues, around 3/4 of the time, the body rejects this egg/sperm combo as being too low quality and it simply never gets to the stage of implantation and comes out with the woman's period. (Thus reducing the 80% to about 20% conception chance per fertile window) This is not a miscarriage, or even a chemical pregnancy, it's a failure of implantation. It happens before any of those things and it is quite common and normal and not even medically considered to be pregnancy at all. As both men and women age, their egg and sperm quality degrade, so you go from roughly a 20-25% chance of conception per cycle to a lower and lower chance the older that you get, that is the mechanism by how fertility declines with age, as well as of course menopause.
But this is why it doesn't matter how often you have sex and why percentage chances don't vary hugely between different couples even if one couple has sex once a month and another has sex every day. The fertile window is approximately 25% of a woman's cycle, but many people prefer not to have sex on their period, (and if you did, you'd know that you were not fertile) so that wipes out another 25% or so, and biology tends to mean that we are more likely to want to have sex during our fertile phase, so the chances of you having sex at a random time and it just happening to be within the fertile window are quite high, probably about 1/2 or 1/3.
Chance of pregnancy with condom used perfectly during fertile window = about 0.1% / 1 in 1000.
Chance of pregnancy with condom used perfectly during non-fertile part of cycle = essentially null
Chance of pregnancy with condom used perfectly, unknown time in cycle = maybe 1 in 2000-3000?
Hmmm OK so I actually came to the same conclusion as you in the end 😂 I think in a roundabout way, we agree, even though I thought that I was saying something different.