The collision of male and female anatomy and the wonder of modern medicine can sometimes be a real pain. Literally. But why? Let's examine.
An Intrauterine Devices (IUD) is a form of female birth control. The IUD is placed through the woman's vaginal cervix into the uterus, where it interferes with the ability to become pregnant. There are two types: one that releases hormones that thicken uterine mucus to stop the sperm and egg from reaching, or a copper coil that causes minor inflammation that kills sperm and egg cells before fertilization can occur.
Whether hormonal or copper coil, the installation process is the same: the main T-shaped body of the IUD is inserted through the cervix into the uterus, with a pair of thin plastic strings left extending through the cervix into the uterus. These strings are usually cut so that around 1-2 inches are left inside the vagina, giving the doctor something to grab onto if the IUD needs to be repositioned or removed.
This is where anatomy comes into the game. The problem here is that the human vagina and penis evolved together, and their averages are pretty similar: the average penis is around 5.5-inches long when erect, while the average vagina reaches around 6 inches fully aroused. Normally that's not a problem; it takes some very specific maneuvering to put the full length of any dick all the way into a vagina, so that there's a bit of IUD string extending into the vagina isn't normally a problem. But the moment you add a longer-than-average penis into the equation, things can get a little prickly.
The problem is that these IUD strings are poorly named. Your concept of a "string" is probably something soft and flexible. But IUD strings are not that — they're semi-rigid solid plastic, and the cutting-to-length often leaves a sharp end. So when the head of a penis is sticking deep into a vagina with an IUD, it can ram right into the pointed end of a stiff IUD string.
And it doesn't help that the head is where most of the nerve endings in the penis are, so poking into an IUD string can be a very sharp pain. You're not likely to injure your penis poking an IUD, but that doesn't mean it doesn't hurt. And there's a chance that impacting with the IUD strings could dislodge the So what are you to do about this?
The most immediately solution is to adjust your technique in bed to use more big dick-friendly sex positions so that you aren't thrusting so deep. A longer-term solution requires your partner make a trip to her gynecologist. While the doctor may protest that there's no way your penis could be contact the IUD strings, their job is to handle their patient's needs. They can cut the IUD strings shorter or reposition them so they're not as inconveniently located. Worst case scenario, they gynecologist can remove the IUD entirely.
IUDs can be a great option for women and couples. They make birth control simple and easy — no pills, no schedules, no tracking, just put it in and you're done. Most of the time they're set it and forget it, but if you're working with abnormally sized anatomy it can be an irritating, but not unsolvable, problem.