The first date is an especially crucial time in a developing relationship. It is when it is decided whether or not there is going to be a relationship. You may only have a couple of hours to get to know this person, and only a limited amount of words to sell yourself to them.
Everything you say on the first date can and will be held against you. Usually, it's the worst of what you say that your date will remember. They won't bother to remember the fact that you volunteer at a soup kitchen every weekend, but they will remember that you were arrested in high school for staying out past curfew.
Certain things that people say while on dates make them look clingy, or even get them put straight in the friend-zone. Watch your words, because you might just ruin your chance for something great.
If you find yourself opening your mouth to say something along these lines, slap yourself. Bringing up past relationships is the poison apple of first dates. It seems harmless enough at first but can completely ruin your chance at getting a second date.
Bringing up your ex makes you seem clingy. For your date, it's a major red flag. Most people see it as you have never gotten over your previous relationship, and who wants to be with someone who is still hung up over someone else.
"I'm doing great/horribly financially!"
First dates are not the time to bring up financial stability. It could ruin your chances of being in a meaningful relationship whether or not you are at a good place financially. If you are in a bad place, people are going to avoid you because they don't see you as being able to support a relationship at the moment; if your in a good place, people might be with you but there is a large chance that they are just in it for the money.
"I want to get married and have two kids named Stan and Sally!"
Now is not the time to talk about what you want to do in the future. Saying anything like this is sure to scare off potential partners. It's to early in the relationship to lay this on anyone, this is more of a six month sort of thing. A better thing to do on a first date is establish what you want in a relationship. Define if you want something casual or if you're hoping for something a little more serious.
"Will You Marry Me?"
I don't care if you have known this person your entire life. I don't care if you have been talking to them on the internet for the past year. I don't care if this is your best friend who you have secretly been in love with since you were six.
You never propose on the first date. You just don't do it. Even if you have been friends with this person forever, you have to see if you're romantically compatible like you are friendship-wise. There is a huge difference between being someone's friend and being someone's partner. Even if you like it, wait a little bit before you put a ring on it.
"I Think We're Going to be Good Friends"
If you ever say this on a date, I'm hoping that you planned on being put in the friend-zone. By doing this you are making it so that the other person sees you as a friend only. I'm sure that you have been put in the friend-zone before, and it's not an easy thing to get out of. Save yourself the trouble. So unless you have no romantic attraction to this person at all, just avoid using this term.
Sex is not an okay topic for the first date. The first date should be all about getting to know each other, not getting to know their body. You need to build a solid foundation for a relationship before you start to bring sex into the equation.