Many employers discourage in office relationships, or fraternization. But dating a co-worker, or being asked on a date by a co-worker, is not necessarily sexual harassment.
If the person asking you out is your supervisor and you feel pressured to say yes, it could be ‘quid pro quo’ sexual harassment. If you say no and the person continues to ask, that could also be harassment. Similarly, if you say no and are then retaliated against in the terms or conditions of your employment, that could also be a quid pro quo violation.
If the relationship is a consensual one between peers, or between you and someone not in your chain of command, it is not likely to be considered harassment (though it may violate a company policy).