What do you think about me telling a girl things aren't working out after our third date, and an hour after we had sex on that same date.
Can you explain more about why you are choosing to do that?
Well, I'm quite concerned about my job and it's good to be careful.
If you are not ready for a relationship, you should be honest about this from the start.  It is wrong for you to lead a woman on just to have sex with her, because she might be truly interested in you.
She can't be disappointed if she already knew that I don't want any kind of relationship.
Not wanting a relationship is one thing but ending things completely after having sex is disrespectful and hurtful. You should reconsider your decision and why you are doing it.