Yes, it is absolutely okay to reject someone before the first date if you feel that you won't be compatible or if you have significant lifestyle differences. It's commendable that you are considering both your and the other person's time and emotions. Being honest and upfront about your feelings and concerns is essential in any relationship, even at the early stages.
It's better to be clear about your expectations and intentions from the beginning to avoid any misunderstandings or hurt feelings later on. If you feel that your values, goals, or lifestyle choices are not aligned, it's perfectly valid to communicate that to the person before the first date.
By expressing your feelings respectfully and honestly, you allow both yourself and the other person to find someone with whom you are more compatible. It's a part of the dating process, and people appreciate honesty and straightforwardness in such situations.
Keep in mind that everyone is different, and what may work for one person might not work for another. Trust your instincts and be true to yourself when navigating the dating world. It's better to invest time in finding someone who shares your values and lifestyle than forcing a connection that may not be right for either of you.