I'm not going to tell my crush that I'm asexual before the first date
Why would you hide that from them?
I believe it's a turnoff and I don't want him to find out about my asexuality via conversation.
I can understand this being a hard conversation for you to have. It would be viewed as being deceptive if you were not to reveal your sexuality to someone that you are planning on dating, this is something you should have a sit down conversation with them as soon as you can. Continuing this relationship while withholding important information like this can cause major issues down the line.
He will date me if I'm attractive.
It might be a deal breaker though if he finds out you lied to him. You should find the right time to tell him about your sexuality, or you might come off deceptive.