When you have closer & more valuable relationship with your Female friend than the individual who is expressing interest in you ... so you tell the person you are a full-on Lesbian to avoid any let-downs or awkward conversations for the remainder of the night. Clear boundaries and low-pressure -- It's the perfect way to stay in a great conversation.
"Yeah, my Lady is amazing too. We're in a Lucid Lesbian relationship. She's the best."
"No worries, that isn't even an issue. I'm a Lesbian, dude. Finish your story!"
"Well with modern dating, it's really common for people to not even live in the Same town.... for example, my girlfriend lives in Sarasota, and I live up here. We love it. One of us is taking a vacation all the time. "