Quite often we hear arguments that sound like the following:

"I can do whatever I want even if it's wrong as long as I'm not bothering anybody", or
"It's my body, I can smoke or do drugs as long as I'm not affecting anyone", or
"Me and her are in agreement, if we sleep together, what's it to you?!", or
"Why do you care if I pray or not, go to church or not, etc...  I'm only impacting my spiritual life"... etc.

Basically an argument that follows a pattern like "I can do ______ (a negative action) as long as I'm not bothering anybody or affecting anybody".  Sound familiar?  The answer to this argument comes in Genesis in the following verses:

"The earth also was corrupt before God, and the earth was filled with violence.  So God looked upon the earth, and indeed it was corrupt; for all flesh had corrupted their way on the earth."  (Genesis 6:11-12)

At a first glance the second sentence might seem redundant.  However, after carefully reading it we see that it adds a new concept.  The first verse simply says that the earth was corrupt and filled with violence.  This could be understood as "the people on the earth" were corrupt and filled with violence.  Then comes the second verse to clarify and make the point that "the earth" was corrupt because "all flesh" corrupted their way.  As in, the earth and the nature were corrupt because the people on the earth were corrupt.  My sin doesn't just affect me.  It affects everything and everyone around me.

We saw this before in Adam's sin, where the ground was cursed because of Adam's sin.  "Cursed is the ground for your sake; In toil you shall eat of it All the days of your life.  Both thorns and thistles it shall bring forth for you..." (Genesis 3:17-18)  Not only was the ground cursed because of sin, but the sin also affected its nature.  Thorns and thistles sprang forth after the sin.  We can't say that the sin affected the person only.

Another dimension of the impact of sin is on people too.  While in appearance the sin doesn't hurt anyone, doesn't bother anyone or doesn't impact anyone, in reality however it affects everyone!  The Holy Spirit teaches us through St. Paul's writings that we are all members in the Body of Christ.  That's what the Church is.  "Now you are the body of Christ, and members individually" (1 Corinthians 12:27)  If we are all members in the Body of Christ, how can a suffering member not affect the rest of the Body?  If you have a wound in your arm, your whole body is affected.  If you have a stomach virus, your whole body ails.  "And if one member suffers, all the members suffer with it; or if one member is honored, all the members rejoice with it." (1 Corinthians 12:26)  We cannot say that our individual sins do not affect anyone.  In fact it's quite the opposite.  Our sins affect everyone and everything around us.  These sins affect the spiritual soundness of the whole body of Christ, i.e. The Church.  My individual sins that I do behind closed doors affects the whole cosmos.

Also just like wickedness corrupts the earth, the opposite is also true where righteousness blesses the earth.  A righteous man blesses the people and the place around him.  It was said that Abba Paul, the first hermit used to get his daily provisions by a raven.  We see in his story that the raven brought half a loaf of bread daily instead of snatching any food from him by its nature.  Moreover, when St. Anthony, the Father of Monasticism visited the St. Abba Paul, the raven brought them a full loaf of bread.  The next time St. Anthony visited St. Paul he found that St. Paul had already departed and wanted to bury his body.  At this point two lions approached St. Anthony in submission and dug a grave for St. Paul.  We see here that the lions lost their beastly nature due to a righteous man.

Our behavior affects everything and everyone around us no matter how secret or how harmless they may seem.  In fact the Church Fathers teach us that one of the reasons behind the incarnation of the Logos is to bless the earth.  Man's wickedness corrupted the earth, while Christ being the source of righteousness blessed it again.