WARNING: PG-13. Have you ever heard someone say, "I didn't mean to have sex, it just happened?" If you'd like, replaced "sex" with "I didn't mean to get drunk and then drive," or "I didn't mean to lie and then have to tell a bigger lie to cover it up."  The concept in all of these is the same: something went from OK to disaster in one step. However, that's very rarely the case. 

Look at the attached picture. The Swiss Cheese model of accident causation is a model used in risk analysis and risk management. It shows that we have multiple layers of defenses against accidents, and that multiple failures, when aligned, can spell disaster. This is also true spiritually. 

Usually people don't just instantly fornicate. Instead, they like each other. They talk to each other. Then maybe they kiss, talk about sex, start spending time alone together, start taking off clothes, touching sensually, and THEN it happens. That's why God warns us, "But put on the Lord Jesus Christ, and make no provision for the flesh in regard to its lusts." (Romans 13:14) Each of those steps toward sleeping with someone is us allowing a provision for sin, but each is a chance for us to stop sin in its tracks. 

James 1 explains it like this: "But each one is tempted when he is carried away and enticed by his own lust. Then when lust has conceived, it gives birth to sin; and when sin is accomplished, it brings forth death." (v14-15) Let's use every chance we have to prevent sin!