Is asking a girl out sexual harassment?

No, at least not the first couple of times. If she makes it clear she is not interested and never will be, or she lets you know she is not attracted to your gender, then if you keep on, it is harassment. Trying to change a woman or get her to compromise her convictions about sex and dating is also a form of harassment.

Just remember that when you are on the clock at work, you are being paid to do your job, not try to get dates.