The coworker having your boyfriend's car as screensaver does not necessarily mean that your boyfriend is cheating on you. It could mean that she is an over obsessive ex-girlfriend as you seem to make her out to be. The fact that your boyfriend plays the relationship as not serious could mean it was really what he said, not serious.
Let things cool off for a little while. He might just be upset that you accused him of cheating. Don't bring it up again unless you begin to see more signs that are really bothersome.
If you bring it up to your boyfriend again and he does not take your concern seriously, then it might be time to let this one go.
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Well from a males perspective I don't want my girlfriend bringing up someone's right to still like me which I have NO CONTROL OVER THAT. Its like your the one on some bull crap because your provoking a situation because the girl is not giving you the respect you think you deserve so your transferring this hostility to him like he has a way of changing his ex-co-workers behavior. The two were human, which means they shared things about each other that has nothing to do with you- so whether the other female has a picture of his car as a screen saver means nothing- it means you again feel like you can make him talk to her in order to make you feel secure when you feel inferior because you don't know the full extent of what their relationship was like,about, and the intensity. Well you should mind your own business and stop bringing the drama to the guy because at this point your being condescending,ignorant,negative and obnoxious. Only a dumb person trying to loose their relationship will try to pry in the past and bring up stuff just to cause an argument to see if the guys loves you more. Its possible even if the female is not telling the truth and the relationship wasn't serious you already are indirectly calling your boyfriend a liar and he should not be talking to you because its offensive to FORCE your self into the past of someone else's relationship because you have low self esteem.
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