What does this mean?

I've got a girlfriend, she insists she loves me and i love her but, on Facebook she constantly likes some guys pictures when they say 'like is you're single' firstly who the hell posts things like that but more to the point why is my girlfriend liking them... she comments on them and flirts with him on the comments... i told her it bothers me so she said she doesn't like him he's a fuck boy blah blah its just funny... then this morning he posted 'like this for a rate and comment a pic whoever likes your pic thinks you're fit' she liked it and commented and he liked her comment... I am supposed to be enough for her why is she constantly liking these guys things like she's single?
What does this mean?
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