The games which contain the scenes with protagonist's female significant other being stolen by another female, should be given this tag.

It's important to note that due to the prevalence of "girl-on-girl is hot" trope, it's common for many VNs to throw in lesbian scenes between the heroine and another female. Most of the time it is not considered NTR due to the protagonist not minding or even encouraging the lesbian exploration of their significant other. Only use the tag if another female clearly has a design to steal the heroine, and the protagonist experiences jealousy and other negative emotions associated with the netorare fetish.