Now I know a number of people seemingly have an issue with that but generally I always have sex on a first date (if we get on). That said I don't go into a date expecting sex, on the contray I usually go in with little expectation, that way I aren't disappointed. Now I know some argue sex on first date ruins everything but honestly, in 13 years that really hasn't been the case. In fact it's actually helped, let me try to explain with personal examples...
1) During sex he farted constantly. Apparantly he gets flatulant when randy. That was various dates dodged.
2) Usually after the sex we become relaxed more thus making the other dates feel more organic and enjoyable.
3) Sometimes a guy says or does something during/after sex that makes you realize he isn't for you e.g getting possesive
4) Sometimes you find new aluring qualities during/after sex that you may have never seen thus affect whether or not a 2nd date occured.
Obviously theres more to relationship than sex but I believe if both parties are interested and actually wanting sex then just do it rather than wait for some 3rd date rule. After having relationships that both started with either sex on 1st date or 3rd, nothing negative has ever affected a relationship by breaking the 3rd date rule
What's your opinon