You don't have to wait until your next period. You have to wait two weeks because that's how long it takes for the embryo to implant and begin producing hCG.
We advise waiting until a missed period for two reasons:
1. It's usually enough time after sex (aka 2 weeks or longer) that a test will be accurate.
2. A missed period is a fairly strong indicator of pregnancy itself.
Yes, if you had sex less than two weeks before your period you're going to have to wait a bit longer and testing right after a missed period won't get you accuarate results.
The best thing to advise is "Wait two weeks."