EDIT: If you're serious about a relationship, LTR, you'll likely wait longer than 3 dates for sex, up to 3 months is realistic. 3rd date sex is high compatibility + high sexual attraction which is rare.

That reason is because within 3 dates, you should be able to determine if you are really into the person or not.

Anything below "totally clicked", means you two won't work out. And if you're looking for a LTR, you might as well dip.

If you're not looking for anything serious, then if you two are "ok" within those 3 dates, have sex on the third, and continue until the inevitable end.

Trust me, if you're really looking for a highly compatible partner, or "the one", you will be able to tell on the FIRST date, the other two are just to make sure some things left unsaid are said, and that you two are on the same page.

Sex is the final thing that unlocks the relationship. Once you have it and you're happy, that's that. A romantic relationship is formed.