The “big deal” (as you put it) is simply that you’re going against God’s clear command forbidding adultery — and eventually it will catch up with you. One of the Ten Commandments bluntly states, “You shall not commit adultery” (Exodus 20:14).
Why did God say this? Is it because he doesn’t want us to be happy or enjoy life? No, not at all. God knows us far better than we know ourselves, and he also knows what’s best for us. He gave marriage to us for our happiness and security. And this is what happens when we commit ourselves to our spouse and learn to put them first, instead of ourselves. The Bible says, “Love is patient, love is kind. It always protects, always trusts, always hopes, always perseveres” (1 Corinthians 13:4,7).
But when we ignore God’s plan and are concerned only for ourselves, eventually we end up in heartache and insecurity. Right now, you think you’ve found happiness — and for a time, it may seem true. But this relationship will never bring you the security and lasting happiness you seek. Even if you married this person, in the back of your mind you’ll always wonder: Can she be trusted?
Don’t continue on this road. Instead, turn to God in repentance and faith, and commit your life and your marriage to Jesus Christ. Then end this relationship, and instead ask God to help you become the kind of husband he wants you to be.