What is sin? It is doing wrong. A simple biblical definition is "lawlessness" (1 John 3:4), which reminds us that sin is measured against a standard: God's law. (There are also more technical meanings of the word, such as original sin or imputed sin, or sin as a power, which we aren't dealing with here).
My impression is that in Western society today, morality is increasingly measured in terms of respect for the rights of other people, and immorality as the violation of those rights. Thus, for an action to be considered "wrong" it must be in direct violation of the rights of another person or group. Individual freedom is so highly regarded that it must not be restricted unless its abuse demonstrably harms someone else.
This shift in thinking is evident not only at a social level but also in the legal system. We hear of lawyers protesting that their clients stand accused of a "victimless crime" and thus do not deserve any penalty. Criminal acts which have been described as victimless (according to Wikipedia) include individual consumption of recreational drugs (especially cannabis), prostitution and solicitation of a prostitute, public indecency, depiction of cartoon child pornography, and not wearing a seatbelt. The victimless nature of consensual sex acts has apparently been a major factor in the overturning of sodomy laws in many jurisdictions.
While our secular, individualistic society increasingly recognizes only wrongdoing with an external victim (human or animal), the biblical concept of sin has three "dimensions." Sin can be vertical (committed against God), horizontal (committed against another person or group of people), or circular (committed against oneself).
The dimension most commonly referred to in scripture is the vertical dimension. There are dozens of references to sinning against God. These include sins of a "religious" nature, such as idolatry (Ex. 32:31-33; Judges 10:10), but also sins which only involve people (Gen. 39:9; 2 Sam. 12:13). There are also a number of references to 'horizontal' sin, i.e. sinning against other people (Gen. 42:22; Matt. 18:15). The circular dimension of sin is less frequently mentioned, but is emphatically stated by Paul in 1 Cor. 6:18-20:
"18 Flee from sexual immorality. Every other sin a person commits is outside the body, but the sexually immoral person sins against his own body. 19 Or do you not know that your body is a temple of the Holy Spirit within you, whom you have from God? You are not your own, 20 for you were bought with a price. So glorify God in your body."
Other references to sin against self include Prov. 6:32 and Rom. 1:24. It is noteworthy that all three of these texts refer to (apparently consensual) sexual sin, since many of the alleged 'victimless crimes' mentioned above are of a sexual nature. Why are these sins against oneself? Partly, as Paul explains, because we are not our own. God, the Creator and Redeemer of our bodies, has called us to set them aside for a higher purpose. As such, he has given us commandments on how we may or may not use our bodies. When we ignore these in the interests of individual freedom, we actually remove ourselves from God's presence and blessing. We rob ourselves of peace and joy.
It is important to realize that there is overlap between the different dimensions of sin. David's adultery and murder were described as sins against God, even though they directed involved only human beings. There are a number of cases where a sin is explicitly described as being both against another person and against God. Pharaoh sinned against God, as well as Moses and Aaron (Ex. 10:16). The prodigal son confessed that he had sinned against heaven and his father (Luke 15:21). Paul declares that a sin against one's brother is a sin against Christ (1 Cor. 8:12).
In fact, every single sinful act encompasses all three dimensions! First, every sin is an act of rebellion against God, the righteous Creator, Lawgiver and Judge to whom all are accountable. Second, any sin, even an apparently "victimless" one, violates our fellow humans. We set a bad example for others, causing them to stumble; and we defile our consciences, making us more harmful to others. Finally, every sin is against oneself because it is self-destructive and separates us from our loving Father in heaven. As Wisdom declares in Prov. 8:36, "He who fails to find me injures himself; all who hate me love death."
Just as society increasingly discounts the seriousness of 'victimless crime', so the church increasingly discounts the seriousness of 'victimless sin' - particularly sexual sin. It is claimed that sexual sin was very low on Jesus' priority list compared to sins such as self-righteousness and pride. However, this is an argument from silence, and Jesus' apostle Paul made it clear that sexual sin has a victim (oneself) and is inconsistent with our calling to glorify God in our bodies.