Sex on the first date is typically a bad idea, for a myriad of reasons. For starters, the sexual double standards that our society holds so dear: a woman who “gives it up” too fast has to be prepared to be called a slut or to lose her chance at a lasting romance with the same person who was just as willing to have sex with her on the same day.
And even if the gent in question isn’t so old-fashioned as to lose respect for you, if the sex isn’t particularly great, one or both of you maybe immediately done with some thing that actually could have had some potential.
Let’s be real One Night Stands happen but they should not happen. There is too much at risk plus you know you are not going to every marry someone you sleep with on day one. You will always be thinking if she can give it this fast how many other times has she done this. She should be thinking the same thing about you to be honest.