That line is barely even developed to the point that no one, but the developer, can really say what will happen, we can give but educated guesses. And, when I've played the game, the Wife and Bodyguard will, for whatever reason become enemies with one another.
Either way, I don't see the problem of the wife potentially having an affair. It could flavour to an as of yet, bland game, and on the off chance the dev decides to implement it, it adds challenge to the game.
As it currently stands the relationship with the bodyguard is basically zero, and you can max out the Wife's relationship and have somewhat 'intimate' relations with her. Having to 'win' the wife by 'defeating' the Bodyguard, would be an interesting story hook for the game, as it's currently lacking one.
That said, the game is definitely drawing itself towards NTR content, albeit with the main character being the bull-like role, rather than the cuckholdee. The way the character interacts with the Husband shows that in the future, he'll pretty much be in control. It'll naturally start off with the husband 'letting' the character see the wife in his presence, and then slowly, but surely the character taking complete control over the situation. This is reinforced by the fact that max'd relations with the wife has her pretty much swooning over the character like he's gods gift to man.