Why do girls always flirt with me even though I'm straight?

This has become a pattern. I end up befriending girls and thinking we're all just friends and getting to know each other. It ends up being that they're actually bisexual and then they admit having feelings for me. It happened in college with one of my closest female friends, at work with one of my coworkers, etc. I do not want to give mixed signals or make them think that I'm into them in that way. I love their company and we are all an emotional support system to each other, but I'm straight and never look at girls in a sexual way. How can I avoid this from happening again? I feel really bad and guilty when they have the courage to admit it to me and I basically have to say that it's not a possibility. What can I do different? Am I giving off mixed signals or encouraging something unknowingly? I seem to attract bisexual females all the time and most of my female friends are bisexual, like probably 80%. Please advise

Why do girls always flirt with me even though I'm straight?
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