Let's say you have strong romantic feelings for someone, or intend to pursue or repair a relationship with that someone (Person A). Before the time is right, you meet someone else (Person B), perhaps on vacation. Knowing full well that you still have feelings for the Person A and intend to pursue a lasting relationship with that original person, you still go ahead and start a temporary relationship with Person B, spending time with him or her and cultivating romantic exchange of feelings and emotion. You know that it's going to be temporary, since you still have feelings for Person A, but you don't really tell this to Person B, because you're enjoying the interchange. As far as Person B knows, you're interested in them with a long-term basis in mind. When it's time to leave or when you're ready to go back to Person A, you break it off with Person B, using whatever vague break-up language needed for you to feel satisfied. You may even blame yourself, or try to make yourself look like a jerk so that Person B won't take it so badly. In the end, you enjoyed a temporary relationship and personal exchange, and now you move on to pursue a relationship or express your feelings for Person A, sure that Person B will get over you in short order and continue their life.
If you fucking do this, you are Satan, and by the power of my sheer determination I will somehow mentally will into existence the Greek Furies and various other demons to chase you down and exact brutal, unforgiving revenge for the poor person you had a temporary relationship with. You deserve no pity, no quarter, and no mercy, and by the power of justice, may you be condemned to suffer ten times the emotional rape you inflicted upon Person B, with whom you shared yourself deceptively. May God damn you to hell and inflict more suffering and pain upon your lying, sinful heart than you could have ever previously imagined. May storms haunt your tread and water turn to poison in your deceitful mouth; may you be shunned by all of humanity for the rest of your pitiful life. You are the enemy, and you are evil. You are subhuman, and you should burn. There is no forgiveness for those who fuck with or manipulate the human heart. None. So perish.