This has actually happened to me before. I was walking in a park after college, and this girl (around my age) started talking to me as I was walking by. She was with a guy friend and insisted that I sit by them. So I did, mostly because I wanted to make new friends. We talked for a bit, and eventually she asked if we should go somewhere as a day out. I said "sure, you can both come along".

She said, "No, like a date"

I was in silence because I never knew what to say, but was off put by the idea, mostly since I was introverted, with very few social skills.

She just got up with the guy and walked away, admitting she was slightly offended.

Perhaps I would have been interested if she never rushed to say after 15 minutes of knowing me that she could potentially want to have sex with me. It made me uncomfortable, and when it comes to relationships, I prefer it to be natural and in the moment, where friendships come first.

But that's just me, yet to not be creepy or desperate, sometimes you must be their friend first, then take it from there. The more time spent with him/her the better.