That's different to what's mentioned in the OP though. The way you described it you're not fantasising about watching her, rather thinking about what she's done in the past. That's totally different.
And it is a porn thing in almost all cases, with a healthy side order of insecurity (and racism in a lot of cases as well), and it creates problems in relationships for three reasons.
Firstly, thanks to porn, many men are totally dependent on watching and not participating to achieve arousal, and have forgotten that real life lovemaking involves mutual participation and pleasure on both sides. That's not good.
Secondly, porn allows men to 'programme' their ideal sexual encounter through the various keywords and categories, and think they can do the same in a real life relationship by ordering their partner to perform in a way that turns them on only, as if they were a robot or a lapdancer. That's not good
Thirdly, it's a manifestation of a desire to control. Desire to watch partners often comes from a place of insecurity, so men think that if they're in control (key word bolded) of how their wife is unfaithful, that makes it easier to handle. That's not good either.
You talk about wanting your wife to have the 'best sex she can', but this is for your benefit, not for hers