1) They won't think any less of you. Stop overthinking what the other person will think. Most people will actually not think much about it at all.
2) Chemistry says all. Not all relationships have amazing chemistry right from the start. First dates are awkward and can cause someone to be anxious and not really themselves. Although first dates may not always start off with amazing chemistry, by the end of the night this might be different and might create a different outcome than what you were expecting ;)
3) Test drive. I'm all for taking a car for a test drive before buying it, aren't you? Also, sexual preferences can vary soooo much. If it's not a match in that department, I say the sooner you know, the better.
4) It's not the reason that things don't work out. If things don't work out, it's definitely about more than just sleeping together on the first date. Trust me even if you don't sleep together it does NOT mean that they will call you or see you again.
5) Everyone is allowed to have and enjoy sex. If you want it, just have it and stop caring about what people think.
6) It doesn't mean you're "easy". Enough with the "easy" and "slut" shaming. Having sex with people does not define who you are in any way. And if the other person thinks that of you, clearly they're too young for you anyways.
7) It keeps you both wanting more. NOT having sex on the first date is NOT the only way to keep them wanting more. Actually, I find it breaks the ice and opens the door to more flirting and sexting, and gets your relationships exciting right from the start.
8) ...It's sex. And hopefully for you, it's good.