What? Anyone can be sexually harassed by anyone else, workplace has nothing to do with it. Sexual harassment in the workplace is egregious because the conflict of interests. E.g. my boss harasses me, but if I complain I jeopardize my livelihood.
Alex3917 is right that sexual harassment outside the workplace is not illegal, but it's incorrect to say it's not "legally sexual harassment." It is sexual harassment, it's just not illegal sexual harassment.
For example, catcalling on the street isn't illegal. It's still sexual harassment, it's still unethical, but it's not illegal.
That's fair. In many developed countries sexual harassment laws extend to any situation where there is a power imbalance such as in education or in housing (landlord vs. tenant).
Unfortunately this isn't true, from a legal point of view; in many jurisdictions, the legal definition of SH is specifically tailored (and constrained) to "the workplace".
Mind you I'm not talking about what's "slimy", "invasive", or what infringes on one's natural rights as a human being or anything about that. But rather, the legal definition. Which doesn't always jibe with what people would or like it to be.