I'm sorry, OP, but you seem to be unaware of the customs of the future. Allow me to enlighten you on one of them, as it appears to be very relevant to your circle of interests:
You see, when a girl in the future reaches fourteen years of age, she is ritually introduced to sex; the future equivalent of sex ed, if you'll understand. But not through such mundane methods as videos or panthlets or outdated early two thousand methods, no.
Who is better at introducing someone to sex than themselves? Yes, Mikuru, when we see her for the first time in the series, has already lost her virginity in a very torrid night with herself.
So, OP, I'm sorry for dashing your hopes, but she was first on herself. As for lesbian sex not counting, well... technology can be /d/evastating sometimes.