But they do get outraged by photographic assistance of people they know.
Imagine your husband was caught diddling himself in front of a picture of your sister? Now imagine what your brotherinlaw is doing late at night with pictures of you from New years last month at exactly the same time... Now imagine they planned it, synchronised their watches, because they were too chicken to suggest swinging or a foursome, or it was a simultaneous test run before they did approach the two of you about the real thing?
It's really our brains that are disturbing, not what we're doing with our hands.
Deanna was not happy when she thought that Reginald was boffing a holographic definition of herself but it's standard that the holodeck is used for sex quite often. There is a (moral) difference between I suspect using innocuous images/appearances of women and men who are imaginary or strangers (Leah Brahms shit a brick when she thought Geordie was screwing her likeness... Yet Chakotay was plain Chuffed. Men are pigs.) compared to rutting with facsimiles of crewmates and "friends".
Remember Mirror Jennifer Sisko being conflicted by Jake treating her like his mother?
Imagine someone dating (regular) Kira because they wanted to see "her" in the intendants incredibly tight pants again after having a mirror adventure and barely avoided being murdered by that homicidal maniac. How happy is Kira going to be about receiving kudos and affection generated because there's a maniac walking around the station sometimes with her ass?
It's really just a question of context.