Sherlock likes the idea of it more than he likes the act of it.
He isn't much one for physical attachments or pleasure, only stimulation of the mind. So when it comes down to it, Sherlock doesn't pay much attention to what goes on while it happens. Sure, he makes all the right noises and touches, but it's just so John does it again in the future, isn't put off by how Sherlock would normally behave during it.
Because, if left to be himself, Sherlock would simply watch John. He would stare and hold on and absorb the sounds John makes instead of participating much in the act itself. But that might give John the wrong idea – Sherlock knows how he thinks – so he opts instead to play his role and do what others have done as he's seen in his video research of the act.
The act is, if you couldn't tell already, sex. Sex with John, to be precise. And yes, the videos were pornography, but honestly, they did nothing for Sherlock but give him pointers, like how you might watch a cooking show on the telly.
So yes, the idea of sex is more appealing to Sherlock than the sex itself. Because while there is pleasure to behold, Sherlock ignores most of it in favor of observing John. The true pleasure, he has found, comes from the idea, the meaning behind the sex, and from John himself.
Because watching John is all Sherlock needs in order to reach his climax. He hardly needs the rest of it. John's movements and breaths and sounds and words are enough.
And then there is, of course, the concept of sex, and what it means.
The idea is highly intriguing. Sherlock likes it best of all because it gives him so much to think on, to work with.
The idea of sex with John is this: they become One through an act. John fits into Sherlock like a puzzle piece and they merge. John fills Sherlock to the brim, completes Sherlock, becomes the piece Sherlock was missing previously. John gives Sherlock his heart, gives Sherlock a heart to have and hold and protect. In moving smoothly in and out, John forms a bond that cannot be broken between them now that the act has been done. It makes them something new, something different, something unique to them and working solely for their own dynamic.
And it's those thoughts that give Sherlock true pleasure, more than the sex, because he likes the idea of John making him whole, of John being inside him, part of him, a missing piece put back in its place every time they have sex, even without penetration, because it is a moment when they are bonded. He likes that John completes him heart and soul. He loves it, even. He knows he will never have it with anyone else ever again.
So that's it, then. That's why Sherlock even has sex, even partakes in such a plainly human event: because of the implications to go with it, the passion and love. He likes those things, and wouldn't trade them for the world.