When I talk about build up for sex, I mean the foreplay. Even if its limited, like a kiss or some groping, or a render where she is mid taking her clothes off.
just so that it isnt as jarring as: talking, click, now they are mid fuck session and she is halfway to orgasm. Personally I like the character moments during the build up, Torr might be totally into it and excited, Candy might say some weird stuff about being owned or something, Tamara might feign disinterest before "acquiescing", etc.
These kinds of stories are more interesting to readers when the MC actually has to work for something, yes being a sex demon it wouldn't make sense for him to have to work hard, but a single render where he says something along the lines of "get that ass over here" would do wonders.