Stats and Mechanics

Lust: This represents Beth's need for satisfaction or gratification, especially sexual satisfaction or gratification. Despite its name, Lust is not simply sexual desire. It can be reduced by indulging in light flirting, nonsexual romantic activity, self-love, and it can also be "burned off" through sublimation, usually by exercise or similar activities. However, sexual activity, especially sexual climax, is the strongest way to reduce it. Risky and transgressive activities will reduce Lust to a greater degree.
Letting Lust get high is problematic for Beth. With high Lust, Beth will be absolutely obsessed with getting her needs met, and at max Lust, only risky and transgressive acts may be available as Beth throws caution to the wind to get her Lust fix.

Suspicion: This stat represents how close Beth is to getting confronted by her fiancée. If this reaches 100, it will trigger some kind of confrontation scenario. Risky and transgressive acts, as well as acts that aren't inherently unfaithful but create the appearance of infidelity, raise Suspicion. The acts don't have to give direct or indirect evidence of the act to Brian in order to raise Suspicion: they contribute to Beth's guilty conscience.
A confrontation scenario will vary depend on when it happens, and what acts push suspicion to the max. These confrontation scenarios may have a way to weasel out of them with Beth's relationship intact, either through deft denials or somehow convincing Brian that whatever indiscretion he discovered is only a one-time transgression. Game overs are always a possibility when confronted. If you want the story to continue as far as it can go, managing Suspicion is important.
Rejecting Brian's affections, ignoring his communications and engaging in risky or transgressive behavior can all increase his Suspicion (even if Beth is not participating in the activity 100% willingly: ****, ****, and scenarios involving **** are possible). Finding safe ways to reduce Lust can avoid this, though it's not always possible.

Karma (Shout out to sexybjgal69 and her story "Attaining Tenure" for the inspiration to add something like the Luck mechanic from her story): Karma represents fate, luck, serendipity.... whatever you want to call it, but it is a resource Beth can build up by performing certain actions, usually actions of nonsexual self-care or kindness to others. Karma can be used to find better options to reduce Lust, or possibly even save Beth when she's about to be (or has been) confronted by Brian. Karma should be treated like a currency. It can be earned by disadvantaging yourself in some way, and then spent later to gain an advantage.

These are the three main visible variables. Choices will not explicitly tell you how they will affect each, but often you'll have some clue as to what they will do by their description. If a choice is available by spending Karma, there will be an indication of that.

Time: Some periods of time will have a Time meter, providing the opportunity to engage in as many activities as Beth can fit before the time runs out and the activity is over. This could be as big as a weekend or as small as the amount of time Beth has to pull off a risky behavior without getting caught.

HIDDEN VARIABLES: In addition to flags for certain chapters that can alter the text of later chapters (and options as well). There are several hidden variables.

Shame: Perhaps the most prominent hidden variable. Shame represents Beth's attitude towards her fidelity to Brian. It does NOT determine whether she will or won't engage in a transgressive act. It determines how she will feel about her actions. One of the character creation steps sets initial shame to one of four discrete positions.
If Shame is below 30, Beth is callous. She no longer cares (or never did) that much about fidelity. She views her relationship with Brian as nice and useful and a path to a luxurious life, but she's not going to wring her hands too much about having fun outside of the relationship, especially if she's not getting the gratification she wants. She is more concerned about the consequences for her if she gets caught than what it will do to Brian.
If Shame is between 30 and 50, Beth is rationalizing. She loves Brian and normally she would care about hurting him, but she will rationalize her acts of indiscretion based on her belief that her needs are not being met, and her fears that her relationship with Brian is going to be like this forever.
If Shame is between 50 and 70, Beth is conflicted. She really loves Brian and really does not want to stray. But, she can't deny her desire, her curiosity and her excitement towards transgressive acts. She struggles with these feelings, and straying will make her feel guilty.
If Shame is over 70, Beth is ashamed. She is head over heels, Cinderella and Prince Charming levels in love with Brian. So when she does find herself engaging in transgressive behavior, she is filled with deep shame (which has the potential to stoke her desire even more).
Shame can move over time. It moves slowly: situations that increase Beth's frustration with or contempt for Brian will lower Shame by a small amount. Situations that give Beth a good reason to feel shame for her actions will make it go up by a small amount. Keep in mind that Shame is a slippery slope in both directions. If Beth has low Shame, she will tend to fall further, and if she has high Shame, she will tend to increase her Shame.
Individual Character Bond: Prominent objects of Beth's possible affection will have their own individualized Bond score, representing the bond and spark between Beth and the character. Higher score will make it easier to commit indiscretions with that character, as well as more serious indiscretions. The only affection object that doesn't have this score is Brian himself. Beth and Brian are engaged to be married: the full range of sexual activities are available (in theory) to them.

Partner: How close Brian is to making partner. Not a prominent variable, but it will have an effect on endings.

A note about setting: I try to avoid outright identification of exactly where, but keen eyed readers will deduce that the story is set in the greater Los Angeles area, as is common in my stories.