Because engaged isn't married. And God's design for sex has never been "the person you love" or "the person you're going to marry." It's your spouse.
Sex is good, in the right context
Sex was made by God, and it's for our pleasure and procreation. It's not dirty, it's nothing to be ashamed of, and it's something we have to talk about. As the church, we can't stick our heads in the sand and pretend that if we don't talk about it, people won't struggle with it.
My wife and I love building a fire, especially after the kids go to bed. It's relaxing, it's warm, it's romantic. But move that fire out of the fireplace and into the middle of the house, and I risk burning down my entire home and hurting a lot of people. Sex is great when it's used in the context it was designed for. Any time sex is used outside that context, it takes something good and turns it into something damaging, harmful and messy.
The context for sex has always been one man and one woman who are married. Not the person you love. Not the person you're going to marry, so it's OK to go ahead now. "Let marriage be held in honor among all, and let the marriage bed be undefiled" (Hebrews 13:4 ESV).
"Engaged is basically married."
It isn't. An engagement is a promise to make a covenant. It isn't the covenant. Engagements get called off. And if you can't wait for a few months now, what makes you think waiting gets easier when you're married and one of you is sick, traveling or just not in the mood? Self-control before the wedding is practice for faithfulness after it.
"That's so outdated."
"There's no way you can actually expect me to stay a virgin until I'm married! The Bible needs to get with the times." First, we serve Jesus, who lived 33 years as a virgin until He was crucified. Soooo…
God put sex inside marriage not to steal your fun, but because He knew how best to use the gift He gave us.
"Run from sexual sin! No other sin so clearly affects the body as this one does. For sexual immorality is a sin against your own body. Don't you realize that your body is the temple of the Holy Spirit, who lives in you and was given to you by God? You do not belong to yourself" (1 Corinthians 6:18–19 NLT). Notice He doesn't say flirt with sexual sin. He doesn't say it's just a little; we're grown-ups; it's OK. He says run.
If you're already sleeping together
If you're in a relationship and you're sleeping together, today's the day to stop. Not just because it's wrong and sinful, though that should be enough. But because no matter the sin, sin always leads to death. It always destroys, and it always hurts.
You're not damaged goods. "If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness" (1 John 1:9 ESV). Starting today, you can choose purity. Here's what that can look like:
Talk about it together, and decide on the line before you're in the moment.
Change the setup. Don't stay over. Don't hang out alone late at night.
Bring someone in. Tell a pastor, a mentor or a married couple you trust, and let them check on you.
For more on God's design, read What Is God's Design for Sex and Marriage?
Scripture: 1 Corinthians 6:18–20; Hebrews 13:4; 1 Thessalonians 4:3–5; 1 John 1:9