See, I get that you want to avoid that for the reasons of "spoiling" her...but we've already established that it isn't real. Again, nobody feels (and that's the most important part) that Sophia fucked the mob guy's brother...but everyone saw it. She fucks him in that fantasy.
And that's all I'm talking about.
So, for instance, if Sam fantasizes about having had the nerve to kiss her pussy in his bedroom you can show that without actually showing it. Allude to it as done in the fantasy.
Again, if the fear is spoiling the moment you can allude to it without spoiling it because the point is the character's fantasy...it's THEIR view so the player will know what's happening without actually having to have the player see it all.
His head in her lap ALLUDES to it without seeing him eating her pussy.
If he is between her legs it ALLUDES to something without showing it (and even that is admittedly a bit more than would be needed).
Neil can press her a bit more in the bathroom or be dealing with a much more intoxicated Sophia without having to show him fucking her with penetration, lol.
So I get what you mean and the character doing it may also matter but I'm not calling for fantasies of outright XXX penetration and such. The allusion is key. The player isn't dumb but it also doesn't trip that "well, now I've seen it so I don't care anymore" line because if that were true everyone would look at fan art and stop caring about the actual story. They look and still care because they don't feel it as being real.