And here I thought California was supposed to be such an open-minded state

I have a lot of friends who happen to be girls. My first best friend when I was really young was a girl, before I moved away to a new town. Right now, I'd say there's a 50/50 divide between male and female friends of mine. I can attest to the fact that just because you have friends who happen to be girls does not mean that you're climbing in bed with them every time you close the door. Yet the paranoia always remains in concerned parents who are, needless to say, hesitant at the notion of becoming grandparents. The solution we've always used would be to hang out in groups. It's not as nice sometimes as having some alone time, but if you hang out with more than just each other, it's a little more obvious that you're not having six (unless for some reason you'd be having a threesome).
At the same time, try to understand where your parents are coming from. I may have a friend group comprised of equal parts male and female, but many of those friends are actually in relationships with one another, and they do have six when they can. It's not uncommon for two friends to become "more than friends" over the course of a single night spent alone. And while you may have no intention of that happening, and though it may never happen to you, your parents do not have the ability to read thoughts and know your true intentions. It's only natural that they should be so distrustful.
Maybe you can tell them you're actually homosixual. Then they might have nothing to worry about