I used to have this idea that that's just what people did. meet, go on a few dates, then have sex after a bit (couple of weeks? a month?). and thats not far from the truth, really, but at one point a new person I was dating said basically "if two people like each other and want to fuck, what's the hold-up?" and I realized there really wasn't one. that you dont really need any rules like 90 days or the 4th date or whatever. just feel it out as it were.