First of all, talk to a lawyer.
One thing that is really overlooked about harassment is that it's not about a single event. Realize a few things:
* She's saying that it's ok because in the real world most companies will protect the man over the woman. Women who want to get ahead learn to never put her neck out too far. All too often, clearly bad behavior will result in "training" for the guy. (After all, she was a hottie asking for it, am I right?) Do you really want accept an apology for her that it's not a big deal that she was harassed at work?
* Workplace harassment happens because most guys know they _can_ get away with it. More importantly: if this was the line cause a stir, the he'll just be more subtle next time. It's boundary testing that is no different from a four-year-old. It _will_ get around, because this guy will associate with others who don't mind his behavior. Do you really want to be known for that type of environment?
* Once the workplace accepts an event, it becomes that much harder to rationalize dealing with equal or less events. You'll have people actually arguing that Henry wasn't punished when asked Sally out, why are you punishing me? And, he could have valid precedent. Do you really want to be dealing with the fine lines between behaviors?
Fundamentally, sexual harassment erodes trust. If you have a workplace that has clear line between humor and personal relations, that creates teams who understand and care about each other: you can joke about sexual innuendo, personal problems, etc. When someone is unhappy with something, he or she feels safe to speak up. Encourage people being humble and considerate. If harassment is allowed, at best it just causes drama. At worst, it causes power problems that kill trust, brings in more people who causes problems, and the real possibility that someone could own your start up by lawsuit.