I’ve never heard of this as being “a rule”. But if it was, I’d say that’s a really good rule, because we are complex beings and the good reasons that many cultures and religions frown on this range from the superfiscial like STDs, and unwanted pregnancy (which can really mess up a potential relationship, even ruin a family, even with the best prevention) to really deep, “spiritial” reasons.
Physical compatibility / chemistry is an important part of a relationship, but it’s no more important than the intellectual, emotional and spiritual dimensions and it’s also arguably the quickest/easiest one — and you don’t need to test it by going all the way, it pretty much works the same for everyone, so keeping it for later on makes a lot of sense — why would you want to be with someone who doesn’t “get” you, just using you to comfort themselves?
The other dimensions of a relationship take more time to build and you have infatuation working for you already, you’d be stupid to use sex to cement your relationship so early on… it’s a sure way to guarantee you running out of runway and the relationship ending… it’s not a matter of “intrinsic compatibility” that you’re testing, it’s more a matter of two people caring for a fragile little patch of land (the relationship) by making sure it gets enough sunlight and water, so that it the flowers in the land can flower… if you drown the patch of land in water on the first date, aren’t you just disadvantaging the flowers and helping on the weeds?… Proving your inability to at self control and your propensity for induldgence and instant gratification. Please name me anything good that can be built on those values…
Here’s a better study for you: How many partners, on average, do the two camps have? How long do their dating and relationships last? How many end in divorce? The thing is — most people think they’re “searching for the right person”. I think that’s a splinter in our brains — and that, without walking a mile with someone who could be the right person, they will never be the right person. It’s about spending more time with someone, you both change through getting to know each other. You both level up. With a lot of brief encounters, you deprive each other of that opportunity.