This thread discusses face-slapping from a consensual, bdsm scene, as opposed to the same in an abusive detrimental relationship.
It cannot be overstated - these two things are not the same.
For some, as part of a consensual scene (where the giving/receiving of such consent has many forms, a complex topic best discussed on its own thread), face-slapping is an expression of a power exchange, dominance, submission, vulnerability, humiliation, or some combination thereof. It's a form of rough play enjoyed by many.
All this being said - as stated by @SwitchableSusie and others - this is a discussion relating to this practice inside a consensual, BDSM-oriented scene. I recognize that for many they would not enjoy this out in scene, and I suspect for anyone here, outside of scene this would be way out of line; generally, in nearly every society (with the possible exception of a White Sox bar), slapping people about is considered a baddish thing. Most of what we do or discuss here would be inappropriate outside of a consensual relationship, from rectal thermometers to restraints to Susie's butt-ka-bob. That is not, however, what this topic is about. Posts like 'if anyone ever smacked me in the face, I'd...' aren't super helpful, and aren't on topic. Sure - if anyone ever smacked me out of the blue, I'd probably call the cops (after...clarifying.....a few things). If I smacked Susie out of the blue, I suspect she'd call the cops. I could never actually hit Playtime, but if I did, it'd probably be the end of our relationship. Again, however, that's a totally different subject having nothing to do with kink.
For the sake of keeping this topic on track - I remind posters that this topic is about a fetish practice, not abuse or violence, and request that posters keep that in mind while posting. Let's stay on-topic please.
Like every aspect of BDSM play, its enjoyment, however, is far from universal. For some, they just don't appreciate this type of play; for others, it invokes unpleasant memories or foul emotions. This is completely ok - as is often stated, ykinmk - your kink is not my kink.
There is a legitimate discussion (part of this topic) to be had when two partners' fetishes do not perfectly line up. In this case, my response to the original poster is the same as I've given nearly everywhere else: communicate. Your partner tells you she's interested in face slapping - you indicate you're not. Both of your positions are 100% valid. Like anything else in the world, the best way to come together is to earnestly discuss the differences in your opinions, with an open mind and a willingness to embrace the other points of view.
She needs to accept that, particularly for those who have not seen or enjoyed it in a fetish scene, face-slapping can be a bit of an out-of-the-box thing to propose. My guess is, if she's interested in face-slapping, she's interested in particularly rough domination scenes. To that end, why not try starting there? Maybe not face slapping, but scenes over time that build up the intensity. Once that intensity is defined as something both sides agree they enjoy, face-slapping will be a simple extension of that, rather than a new topic out of the blue.
You also need to be willing to view your partner's needs and desires with an open mind. She has wants and interests that she wants to explore - and she trusts you enough to ask you to help her explore them. Be willing to try - be open, be honest. You may learn you enjoy things you never thought you would. You may find you don't necessarily enjoy some things, but love how they make her feel, and want to keep doing them for that. You'll need to share feelings after sessions; learn. Sometimes you might find something really hot and want to explore it further - sometimes you might find things too intense and need to slow down. All of that is ok.
At some point, after many scenes and discussions, you will both come to understand each other's position, and, with any luck, reconcile your wants and interests. Good luck, have fun, and stay kinky 😃