I think you're worrying about nothing—women don't care half as much about penis size as men do!
Recent studies have shown that the average penis size is actually around 5.8" when erect. However, that's the average, which means around half the population will be smaller than that.
Nobody worth your time will think you look foolish because of your penis size. Besides, would you really want to be with someone who cares that much about something so superficial? Being a considerate lover is what matters, not how big you are.
The only times I've ever heard girls laugh about a guy's penis size is when the man in question has done something bad, like cheated on them or treated them really badly.
I don't know anybody who makes relationship decisions based on penis size!
However, if you're conscious about it, you're less likely to enjoy yourself in bed, and your partner is going to pick up on that. She may even think that you're holding back or seem uncomfortable because there's something wrong with her.
The best thing you can do is stop worrying about it. Worrying won't change the size of your penis, but it will affect your enjoyment of sex. Relax, and stop worrying!