Love is different from LUST. If you love someone and respect her dignity, you should marry her first. But if LUST comes first, the reason why you marrying her is because she's pregnant. SHUTGUN marriage is what they called it.
This is all rubbish. There is nothing wrong with having sex, if the couple is serious about their relationship. In fact, I would advice them to delay their marriage for 5-6 years, to find out whether they are really compatible to each other. And it will be foolish to abstain from sex during this period, as humans are expected to indulge in sex during the 20s. If you wait until the 30s to have sex, then by that time your libido will be gone.