Seeing as we don't know his sexual orientation, you don't know whether he would have made those advances if she were not a woman.
And no, a man sexually harassing a woman isn't always sexist (even though I'd imagine that in the large majority of cases sexism plays a role).
Sexual harassment ≠ Sexism
What's important to discern is the reason behind the harassment. If a man is harassing a woman because he views his gender as superior to hers, or that she's nothing more than a sexual object, then that's a pretty clear-cut case of sexism. But if a man acts inappropriately towards someone because he lacks self control/tact, has poor social skills, etc. and is only targeting that person because he's attracted to them, then it's sexual harassment.
And no, a man sexually harassing a woman isn't always sexist (even though I'd imagine that in the large majority of cases sexism plays a role).
Sexual harassment ≠ Sexism
What's important to discern is the reason behind the harassment. If a man is harassing a woman because he views his gender as superior to hers, or that she's nothing more than a sexual object, then that's a pretty clear-cut case of sexism. But if a man acts inappropriately towards someone because he lacks self control/tact, has poor social skills, etc. and is only targeting that person because he's attracted to them, then it's sexual harassment.