Do you think people that say they are only looking for a short term relationship, are people that already have a partner?

Think about it? I know some people just want sex and nothing more , but why a short term relationship instead of a hook up or FWB’s. To me it sounds like they are basically saying , I am just going to have sex with you for a little while , until I get bored of you or so my partner doesn’t find out? What’s your thoughts on this? What does short term relationship mean to you?
Do you think people that say they are only looking for a short term relationship, are people that already have a partner?
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