What, you mean Falling Undercover?
There's nothing inherently wrong with the use of silhouette scenes, but they should never, ever be used for important scenes unless there's a setting for it. Silhouettes should be used in situations where the player either already knows what's going on but the MC doesn't, or in situations where the characters are not important. The example I would use is, once again, Henteria Chronicles 3. There's a point in it where you're in a military encampment and the various men and women are hooking up because of the danger they're in. They're either all silhouettes or just grunting and groaning out of sight. This works because they're nobodies, maybe one of them exists has a role larger than simply existing to have sex. Yui's first time was far too important to be wasted as a shadow on a tent wall and I'm still surprised that Inatari thought it was a good idea.
The only way I'd accept silhouettes being used is if the game employs multiple POV settings. Which is to say if you can pick an option to have your POV be limited to only what the MC sees or having an omniscient POV that sees everything that happens even if the MC isn't present. In a situation like that a silhouetted sex scene makes sense.