It's the look in her eyes. How could it be anything else? It's a look of defiance that could be directed at the hunk who's about to spank her (hard, if his musculature is anything to go by), or at the viewer who has intruded. The hunk and the fishnets put it over the edge for me, but only after I realized that there was an implied third party: she's not looking at the guy who's spanking her; she's looking at whoever has dared to watch her getting spanked.
Who would the hottest possible viewer be? There might be several answers, but the heat gets turned up for me at least, if the viewer is in some way the same person who decided she should be spanked by the hunk. Marriage is kind of the go to for me, in that regard, but I can imagine an equally hot FMf version of the scene, in which a dominant partner in an Ff relationship has decided to punish her sub with a kind of ultimate humiliation in having a burly guy do it.
Fishnets are just fishnets; cf. lace: anything that covers but doesn't really cover has the double hotness of shaming the sub who's wearing it and providing the Dominant with the voyeuristic pleasure of looking at the sub's shameful bits.