Netorare, Netorase, and Netori are separate things. They're connected in that the object (target of affection) has sex with somebody other than their loved one, but they all have completely different emotional objectives. These tags aren't beholden to each other, so beginning a story with netorase doesn't mean netorare is necessary or welcome. Obviously, stories can have mixtures and variations of the three, but they each have their own functions. This is why people specify instead of just using a blanket term because sharing for fun, or taking to feel macho, are completely different from getting robbed and emasculated.
Just to specify: Netorare - Getting stolen from, Netorase - Sharing, Netori - Stealing