Humans tend to be tribal. Also the higher primates too. Chimpazie females regularly venture out of the clan to mate with the rival clans bigger more dominent males. So there's that, Why wouldn't Nat if given permission and encougement to do so, try something with a more obviously gifted sexual athlete, if she chooses to?
I was addressing the trope more broadly than in this specific story. For actual people, there's all sorts of things we're both conscious and unconsciously thinking and feeling when we react.
One of the more notable things in American life in the last 15 years or so was the success of the LGBT movement helping people understand that sexual orientation isn't a choice - that you don't really consciously control whether you're attracted to men or women or both
My personal theory, and I think that reality bears this out more broadly, is that this is true of sexual attraction and arousal in general. And, yes, that includes people whose attractions/arousals are completely unacceptable in society. The exact hows and whys these things happen are a combination of conscious thought, of how we're socialized to perceive those around us, and even more base urges and instincts.
People only have two choices when it comes to the things that arouse them:
They can choose whether to act on those things (relevant for people who're aroused by things that are illegal/immoral)
They can choose whether to focus on specific things, because focusing attention on particular concept or fetish or the like will tend to ingrain it deeper and make it more prominent in broader environment of what gets them going.
But whether they get going in the first place is not really something they can control very much.
When people get specifically turned on my the interracial NTR element, its a cocktail of all of those influences, and they don't really have a say in the matter. You don't consciously control your arousal beyond your ability to choose to think about things you know work. Whether or not they work is not a conscious-level choice.
A lot of stories, like this one, play on the reader's subconscious reactions and broader knowledge to incorporate things just based on reference to the trope. Darrel, for instance, is not an asshole, and doesn't actually appear to have any particular intent to 'steal' Nat or break up her marriage. But NTR fantasies are always fueled in part by channelling the fear and anxiety of that happening into sexual arousal.
I describe it as emotional masochism.
