I use non-con for fics where, for instance, a character is forcibly kissed/touched sexually. This is a non-consensual act, but since it's not "sex", all the connotations for the word rape would be misapplied. It's just as wrong as rape, but I think the word non-con is more appropriate.

Also, considering how much rape fantasy is written and labelled as non-con, I don't think that it's really a euphemism. It's a word with different connotations, used for a fic where rape has different connotations because it's a fictional fantasy.

And really, how is it even a euphemism in the first place?!?! Non-consensual is not anything tolerable! Euphemisms are supposed to make something sound acceptable, not be blunt about an utterly wrong act.