In any sexual relationship no has to mean NO. It does not matter if that relationship is heterosexual or gay. Your friend has to learn this, especially if she wants to remain friends with you.
I would sit down with her, say at a coffee shop or in the Mall and say to her: Look, I do not have a problem with your sexuality. Your my best friend and I would like to stay friends with you. If you want to stay friends then you have to stop trying to hit on me or have a sexual relationship with me. I just don't swing that way. I'm not gay or am I bi curious.
Then as the saying goes "The ball is in her ball park." If you have not told her flat out in words that say you are not interested. She will continue to try if she has a sexual interest in you.
I am proud of you that you for not holding her sexuality against her. She has no more control over her sexuality than you do. But she has to learn that her being a lesbian is not a choice that someone else can make. That some of those women she might want to have a relationship with will turn her down. Some will even shun her because of her sexuality.
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