"...they’ve been told to abstain from pretty much all sexual activity until marriage without a true legitimate reason as to why."
Umm, how about:
• VD (STDs) - Many of these little gems are incurable; do serious damage to reproductive organs, especially hers; some, such as herpes and HPV (warts), aren't prevented by condoms
• Pregnancy outside marriage
• Impeded bonding with husband if she eventually marries - she will be comparing him to others she has had sex with
• Aren't the above enough "why's?"