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?
That's harassment. Sexism is believing or acting as though one sex is intrinsically better than the other, as racism is the same with race. Sexism is not directly related to sexuality in the "having sex" sense, any more than racism has to do with sprinting.
The negative effects of this form of racism & sexism are fairly well documented.
http://blogsallie.4cforchildren.org/2012/02/13/census-bureau...
Do you not think there are grey areas? Areas where a comment might seem harmless but end up insulting someone?
To me the only way addressing this is to speak up when it happens. If you don't then the grey area remains grey and it is likely a behavior to be repeated.