Written by Emily Nagoski, Ph.D.
1. Most commonly, you'll experience pain because of lack of lubrication, resulting in friction. Not enough foreplay or a lack of correlation between mental arousal and physiological arousal or hormonal changes can all result in not enough lube and if you're engaged in penetration for a long time, your body just can't produce enough lube to prevent friction and therefore tearing and irritation. Solution: at least 20 minutes of highly arousing foreplay, plus water- or silicone-based lube (or oil-/glycerin-based, depending on various health issues). As I say, this is the most common reason for pain with penetration, so if you're experiencing pain, start here. If this doesn't help, consider the other reasons.
2. Infection is an underrated cause of pain with penetration. Either a urinary tract or reproductive tract infection can cause pain, and may have no other symptoms. If your arousal is good and you're using lube and still have pain, check with a doc re: infection.
3. Contraction of the pubococcygeal muscle (PC muscle, pelvic floor muscle, or Kegel muscle) can result in pain with penetration. It's the sphincter of muscle around the mouth of the vagina. If you're tense, nervous, anxious, stressed, fearful, unsure or even just inexperienced, the PC muscle may be tensed up, causing pain when you attempt intercourse; it may be so tensed up that you can't even accomplish penetration (vaginismus). Or it may simply be tense and you have to take some deep breaths and allow it to relax with penetration.
4. The hymen stretching or tearing can cause pain. PSA: people who've never had intercourse might not have a hymen, due to, gosh, anything! Tampons, masturbation, sports, lots of things. A small number of people have problematic hymens, septate, microperforate or simply a bit thicker and more resistant than average. If you think this might be you, see your ob-gyn.
5. Pain-related sexual dysfunction (like dyspareunia or vulvodynia) are fairly rare, affecting (depending who you ask and which diagnosis you're assessing) somewhere between .0005% to 12%. If you have pain with the lightest of contact (not just with penetration) on your vulva, introitus, vestibule or clitoris, see a doc.
As you can see, the answer to pain with intercourse is either "lube and foreplay," "really want penetration more before you try it" or "see your doctor." Only when a medical professional actually looks at (and usually also touches) your genitals, can you find out for sure what's up.
A more difficult question is: why would someone have vulvovaginal pain and NOT see their doctor? If you have pain your elbow every time you threw a softball, so intense that you had to stop playing the game, you'd see a doc, right? If your stomach hurt so badly after every meal that you had stopped eating, you would seek medical attention, yes?
So why, if your genitals hurt with penetration, often so much that you were disinclined to have sex, would you not mention this to a ladyparts medical provider? Because you shouldn't be wanting sex anyway and the pain is god's way of telling you that? Because a gynecologist is going to judge your genitals...?
I wish the medical community could get their shit together; it would make my life as an educator MUCH easier, because then I could tell people, "Ah, for that problem, chat with your medical provider," without adding the caveat that I know (I mean, I KNOW) that the medical community is not reliably sex positive, so you have to advocate for yourself, ask questions, even educate the provider about talking in a sex positive way to patients. But you also do have to GO.