The most likely cause seems to be a malformation of the uterus. Some shapes can be worked with, others not, but it certainly wouldn't be discovered until she had problems conceiving. And it's the most invasive to fix, leaving her with the option of saying "no" with more force.
I searched "female infertility" with and with out "non-hormonal" and "female reproduction issues." Like I said, I'm pretty sure this is the best "worst" option, but if someone has something better, I'd love to know.
ETA: An untreated STI seems to be the most popular and likely candidate. My poor girl. Any other ideas? ::looks hopeful::