Is being a side chick ever a good idea?

There’s this guy at work who likes me but he’s in a serious relationship with his girlfriend who also works the same shift as us and at the same place. So one day I was heading to the back to go get something and he wanted to kiss me even though his girlfriend was working with us that day also be she was doing something on the other side of the building. Then there’s this one time I was was working on something in the back and he was behind me leaning against a table and out of the corner of my eye I caught him checking me out and his girlfriend was either working on something at the front of the building or working on something on the other side of the building.
Is being a side chick ever a good idea?
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