Sure, its only a holocharacter, but still...are you a cheating bastard???
Yes.
Just because it's a hologram, doesn't change the fact that you are fucking somebody who isn't your wife.
A hologram may only be a simulated human being, but it's still a human being. Doesn't matter that it isn't sentient - if it looks like one, walks, talks, fucks like one, then effectively it IS one. And even so, as we have all seen, some holograms *are* sentient, such as the EMH and Vic Fontaine.
The way I see it is, if your sweetie is in the holodeck fucking some random hologram, then obviously they don't want you, otherwise it'd be you in there with them and not the holo-skank.