Perhaps it was a clash with a second keyword you had.  Did you have teen or child in the keywords as well for some reason.  There might be more 'disallowed' pairs .. young and sexy? - I'm not sure what they all are but know there are some.

More people are reporting issues with this lately, on the SS forum, it seems something is on. Either a bug, of which SS has plenty, or new uncommunicated restrictions, of which they have plenty as well.

This is just a wild theory and my be way off the mark, but when Google image search did their last big overhaul one of the things they tweaked was to show less "adult' content in image search.  I know on my own small sites a good portion of traffic comes through image search so I assume it has some effect for the big sites as well.

I made a point of going back through and deleting the key words sexy or adult (I used to always use adult, child, senior etc for appropriate ages) wherever I found them with goal of making sure all of my images would show in the safest search settings.  Maybe Shutterstock is trying to give the newer images a better chance to appear in Google image search by making sure they don't have those trigger keywords unless it's in fact an "adult" image.

i reckon it should be banned, along with "cute".
how many times do you see "cute kid" and the kid most certainly is not generically "cute". i could say, as a mother, all kids are cute in some way, so it's redundant.

it's very frustrating to search for "girl" and the first page is full of "sexy" women. The keyword "girl" is also wrongly used. and the word "sexy" on some of those images is just wrong. "s7utty" "sleazy" "horny photographer" may apply more.

lol, +1 to that.