We would not call it sexism, as that's reserved for unwanted attention between the sexes, usually male-to-female, but sometimes reversed... but it would still be just as wrong.
I think we don't have more cases like that because LGTM people are in a minority, but that doesn't mean that an LGTM person can't make unwelcome advances upon another person; why would it? Of course they can.
Can you name something inappropriate from one gender to another that isn't sexist? Consensual sex with your wife is appropriate. Getting slapped on the ass by your SO is appropriate. Getting fondled at a concert by a member of the opposite sex..? Did you want that to happen? You did? Great! Appropriate. You didn't? Inappropriate. Clean and simple.
What else do you feel is sexual harassment but not sexist? Where is your line?