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I could give you a better idea if you send Private Message me a picture of how you usually look and dress. Without knowing how you look, all I can do is guess that if you are off and on with the same guy, maybe people figure you don't want a real relationship, especially if you have sex with your on and off boyfriend.
It doesn't matter how serious you are with this guy that you are "casually on and off with". If you're having sex with someone regularly, you are off the market, at least for a romantic relationship, as far is every guy is concerned.