It amazes how many people are discussing their experiences of infidelity - mostly as the innocent member of a relationship where their partner has cheated. Obviously this is not new behaviour - I've been counselling couples for nearly 30 years, and many of them complain of the hurt and trust lost from the impact of infidelity.
I've come to understand that the basis of a relationship is love, born of trust and communication. Honest, intimate communication. Somehow, these relationship skills are not taught to us in our youth, and for the health of families I think they should be... We get taught communication skills for business, but why not relationships? Communication in business is polite, and not necessarily honest, so these skills aren't transferrable into the bedroom, frankly.
Infidelity is born of shadow self, that wild part of self we don't like. Someone contemplating a tryst outside of marriage needs to question what they are missing, and seek competent support to analyze that... help is cheaper than divorce.
As for sex on a first date - well - I have never counselled a person who really was 100% glad they did it... too risky (as in unsafe). Enough for now.