(Let's assume neither party in any of these situations is a boss or has any power over the other person).
1 A male coworker catcalling or making unwanted statements about a female colleague's appearance to her as she walks by. Is this sexual harassment?
2 A male coworker giving repeated, unwanted invitations to a female colleague to go out for drinks, dinner, or dates. Is this sexual harassment?
3 Do you feel sexual harassment is a fireable offense?
One is too vague. 2 is only harassment if she specifically told him to stop. Three again depends on the definition. Some sjws would consider saying you have nice eyes to be sexual harassment which obviously shouldn't be fireable.
The word "unwanted" implies that she has rejected the man repeatedly.
No it doesn't.
And there you have it folks. "Unwanted" doesn't mean "unwanted." And we wonder why some men have difficulty not sexually harassing people. ---