The one who is left behind... 

Needs to get support around them asap, and work on healing their emotional wounds as a priority. Being cheated on is not a measurement of your lack of worth or your lack of sexiness. It's an act of cowardice from the cheater. On the plus side it's freeing you up to meet someone who is a better fit. 

What happens to the homewrecker?

Insecurity. What if the person you scored gets bored of you? You'll never be able to trust them. Ever. 

So what is the attraction of getting involved with a married person?

The home wrecker is someone who wants the marriage, wants the home, wants the family. But deep down, they don't believe they are good enough to have it. Perhaps a part of them is so disappointed by relationships that they now have stopped believing it's possible. So what better way to affirm that than by breaking up someone else's marriage? 

Some people are just addicted to drama. An affair is illicit, sexy, exciting. It is a far cry from brushing your teeth together, doing laundry, cleaning the toilet and nursing someone when they are ill. It is like having the meat but not the veg. It's fun for the body, but not for the soul. 

Ultimately it is a corroding existence. What's really going on with Sienna is a bucket-load of guilt and self loathing, masquerading as a fear of being shunned by others. She knows what she has done is wrong and now she is looking for some validation. Everyone knows how hard it is to find love, especially as people don't work at relationships as hard as they used to before divorce became so commonplace. It's not surprising that people can be so disappointed by relationships that they will compromise their own values in order to have a relationship, any relationship, at any cost to others. The Home-breaker is desperate for a relationship that offers security and desperation is ugly. 

If you are thinking about having an affair...

Don't! Find a way to end the relationship without being a jerk!