Sex before marriage is sinful even if you love each other. Being in love doesn't alter the reality that the sexual act is outside of marriage, marriage being the ceremonial union of a man and woman in matrimony. Sex outside of marriage is called fornication and the Bible definitely condemns it.
- Matt. 15:19-20, "For out of the heart come evil thoughts, murders, adulteries, fornications, thefts, false witness, slanders. 20 These are the things which defile the man."
- Acts 15:29, "...that you abstain from things sacrificed to idols and from blood and from things strangled and from fornication; if you keep yourselves free from such things, you will do well."
- 1 Cor. 6:9-10, "Or do you not know that the unrighteous shall not inherit the kingdom of God? Do not be deceived; neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor homosexuals, 10 nor thieves, nor the covetous, nor drunkards, nor revilers, nor swindlers, shall inherit the kingdom of God."
- Heb. 13:4, "Let marriage be held in honor among all, and let the marriage bed be undefiled; for fornicators and adulterers God will judge."
So, very clearly fornication is condemned in scripture and there is no provision for it being acceptable just because someone loves another person. Besides if this couple is so much in love, then why don't they carry out what true love is and marry each other? True love is when you put another person first, not when you take something from them. When a man and woman engage in fornication they declare that such love for the other person is more self-centered, rather than other-centered.