Maybe she chose someone more experienced because her previous attempts failed too. Or not! We don't know.
Right. She may or may not have attempted intercourse before; she may or may not have been sexually assaulted. It certainly seems she holds a lot of shame around sex, perhaps stemming from childhood indoctrination (since she's just out of childhood).
For example, members of the LDS (Mormon) church are strongly told that masturbation and premarital sex is wrong, sinful. I've seen videos where Mormon leaders (bishops) are seriously and urgently preaching that masturbation is akin to murder, just as bad as murder, because doing it "murders the soul." Fooling around with your bf or gf is seen as a sin, which must be confessed to the bishop. It can take much penance to be forgiven, and can prevent being "sealed" in marriage in the church, and lead to social ostracization, and of course, Mormon hell.
Some young Mormons hold such negative impressions of sex that even when they do get married, they can't have sex, or enjoy it if they do. Prior to getting engaged, some/many conservative Christians and Mormons are only allowed to go on big group dates. After becoming engaged, they might kiss, but the degree of this varies. Then they are just supposed to jump into PIV on their wedding night, after years of repressing their desires. You can see how scary that must feel.
I find it important to know why people do poly, and in this case, it's especially sensitive.
In this case, by "doing poly," I guess you mean dating a married man who is in an open relationship. Maybe she wants to keep her independence, and prior to this she was made to only date young men from her church, where everyone is meant to marry young and start having babies. That's just speculation on my part.
You mean well, but this could be perceived as pressure to perform, like using a dildo is something she has to do or should have been doing, so just be gentle in how you approach this. Maybe she has no reason to use one for pleasure right now. I don't think I had one at her age.
When I suggested a dildo of an average-penis size, I didn't mean for Steve to suggest Kaitlyn use it on herself. I meant, during a sexual encounter together, for Steve to try inserting more than one finger, then this smallish dildo, before trying to insert his large penis. (However, his last attempt at intercourse with her was rushed and barely involved foreplay, unfortunately, with negative results, causing more disappointment and probably more shame.)
Tinwen, if you look at the wiki on vaginismus, and read around the internet about it, it's a condition that often requires medical help, psychotherapy, and physical therapy, including "reverse Kegels," and using dilators of increasing size, starting with ones about the diameter of a finger. You can see from the list I posted above about degrees of spasm and reaction that Kaitlyn is either a 3 or 4 on the list of severity. That's why I suggest her getting in to see a doctor about this. However, with her degree of emotional discomfort, I am guessing it would take some work and a lot of support to even get her to that point.
On the other hand, many people can greatly enjoy sex without PIV. There are many wonderful things to do that don't involve penetration. As a pansexual, I am all in favor of "outercourse." Perhaps this couple could just take PIV off the table, take the pressure off, and enjoy what is working for them so far, kissing, grinding, fondling, oral, etc. Heck, even cuddling naked in a bed beats making out in a truck.