The reason for the tag, is this site used to have an "NTR" tag. Downside is, the community (not the site's rules) have thousands of definitions for what NTR is and is not. Since there is no tag for sharing/swinging, this game got the NTR tag (since at the time, it fell under the "community's definition" of NTR).
Well since then, the site has changed the NTR tag to now "netorare". But every game that had the NTR tag before, is now using the netorare tag instead. So that's why it has the tag.
There is no actual netorare though. It is just sharing/swinging.