1) It's always easier to have a relationship with someone you're friends with first. You're not in that "trying to impress" stage that happens early on. The girl knows you, your likes, your quirks, whatever. Normally it takes about a year and a half for a couple to "really know eachother," whatever that means. If you're long-time friends, you already are past that stage. There's less bullshit. There's less surprises. You know what to expect, you know how things will go. Your personalities aren't the "early stage dating" facade that people put out there. Your guard is down.
2) I'd say, I usually expect to have sex mid-sentence while asking them out. I've never asked out a girl I didn't expect to have sex with at some point. As for when... it just happens when the time is right, when the mood is right, when she's drunk enough [kidding! (mostly)], whatever. I dated a girl that wanted to wait for marriage, and we didn't have sex... we had other relations, but not sex-sex. I was willing to wait until marriage to vulcanize the whoopie stick in the ham wallet. Virgin or not, both people have to be into it for sex to work. You can't hump sandpaper. So I was always willing to wait... rarely was it longer than a few dates.