We need to be on guard. Proverbs 4:23 is an excellent verse to keep in mind:
"Keep your heart with all diligence, For out of it spring the issues of life."
When I was a boy we lived on the family property in upstate New York. The old farmhouse had spring water that trickled into a reservoir in the basement from a pipe that began 300 yards away. Up on the mountainside, just below the dirt road, was the spring-house. It was a little cement cube with a wooden roof over it. Inside, the spring water bubbled up from the ground and flowed into the pipe that led to the house far below. The low wall was built up to prevent groundwater carrying dirt or pollutants from flowing into the spring from the road just up the hill. The roof kept the leaves from the forest from blowing in. The whole spring-house was supposed to keep bugs and animals out of the water that we wanted to use.
One day the water stopped flowing. I remember going with my dad to check out the spring-house. Dad took off the roof and looked down into the spring with a flashlight. He pointed out the pair of frog legs sticking out of the end of the pipe - blocking the flow. Dad reached down and pulled the frog free and the water was restored (though I imagined it was a bit fishy tasting for a while after that.) While we were there, dad fished out a few leaves and sticks that had fallen into the spring, then put the roof back on. He showed me where it would need some repairs later.
Our heart is like a spring of water. If you want it to flow clean and pure, you need to tend it. Clean out the leaves that fall in. Put up a wall high enough to stop groundwater from running in off the road. Repair the roof periodically to make it effective in keeping critters out.
Moral failure is all around us. It is celebrated in the media and modern entertainment. Lust throws out a snare in modern fashions, in advertisements, in magazines, on TV shows and certainly over the Internet. It is reaching out for us all with the promise that "stolen water is sweet and bread eaten in secret is pleasant." (Proverbs 9:17)
But the real problem is our hearts. We need to have pure hearts. The sin comes from inside of us, not from outside. The Bible says, "Each one is tempted when he is drawn away by his own desires and enticed." (James 1:14)
We need to be on guard - keeping our hearts as pure springs. Do what you need to do to protect yourself. Be careful what you read, what you click on, what you gaze at. Sometimes you need to close your eyes. Every day you need to pray for God's cleansing and protection.
(1Corinthians 10:12) Therefore let him who thinks he stands take heed lest he fall.