I'm late to the converstation here, but basically:
The title "Sexual Game Mechanics" or anything along those lines is kind of misleading, because game mechanics can't inherently be sexual. What they really mean is sexual content as a direct result of an action, and that whatever that action is, is relevant to gameplay. If you've played a battlefuck game like Karryn's Prison, just think of that. You do a certain action, it does something in gameplay and also shows sexual content.
A 'regular' h-game has an overwhelming majority of it's sexual content be shown as long-term results, which usually is problematic because it can be a very long time between those milestones. If a player goes too long without any sexual content, or more specifically, without thinking about sexual content, they'll lose arousal. Basically, having gameplay actions that show sexual content allows players who are engaged with the gameplay to still be aroused. That doesn't mean there can only be one or the other in a given game though.
Sex-actions can also be used to better represent certain fetishes through the interactivity of gameplay. We've all seen someone ask "pregnancy mechanic or pregnancy ending" before. Of course some fetishes can't usually be translated to gameplay like feet or armpits in a way that makes them unique to other mechanics.