[Sunny_01]

I dont think it is always the case at all.

You could be in a relationship that is on its last legs, you could be hoping that it will survive but then along comes someone who sweeps you off your feet. You might well cheat on your long term partner for this person and still hang onto the familiar for all sorts of reasons.

Also what if the spark has gone out of a relationship yet you still love the person dearly. You could end up sleeping with someone else but still love your partner!

There are a lot of people out there who go through life cheating on one person after another then there are those that do it like Kiz said because of circumstances.