I have been watching videos on people talking about having long lasting FWBs. Essentially, these FWB relationships would last 2, 3, 4, 5+ years, and every time that person got out of a relationship and needed sex down the the line, they would contact the FWB and have sex with that person from time to time.

The FWB isn't actually a "friend" that they talk to or hang out with. It's just someone they contact when they are horny. After the deed is done, there is minimal contact until that person wants sex again.

When the person is in a committed relationship, they don't contact the FWB at all until the relationship is over.

Is that wrong? It's sort of like entering a relationship in bad faith, because you are keeping someone else around if it fails... and it's sort of like having a backup plan just in case. Albeit, this backup is just for sex purposes only. What are your thoughts?

Would you be upset to know that the person you are with had someone in their phone for sex purposes should the relationship fail?