Okay, I had to read all 831 posts to get here, for science, and finally, I can comment that we're all missing a very important consideration concerning the rules-as-written.
Energy Drain (Su) A succubus drains energy from a mortal she lures into an act of passion, such as a kiss. An unwilling victim must be grappled before the succubus can use this ability.
Let's break it down.
Part, the first: drains energy from a mortal she lures into an act of passion, such as a kiss.
dictionary(dot)com defines passion as
1) any powerful or compelling emotion or feeling, as love or hate.
2) strong amorous feeling or desire; love; ardor.
taking the first definition, anytime a succubus threatens a character's life, one could say that the character's strong instinct for self-preservation would classify as acts of passion, meaning the succubus could energy drain just by threatening you with death or injury, resulting in your willingness to passionately act in self-defense
I don't think this really fits in with the concept of the succubus, and I sincerely believe others would generally agree.
Therefore "an act of passion, such as a kiss" implies, IMO, that the act must be one of sensual or sexual nature, as per the second definition. Read: an act of passion likea kiss
Following this logic, I don't think that grappling a creature, even a succubus, with the intent to cause them harm, qualifies as an act of passion comparable to kissing. Certain actions during the grapple, such as grinding, gyrating, or giggling, perhaps; but despite its name, a rear naked choke is in no way sensual. Although I suppose that given certain intent, it very well could be.
Part, the sequel: An unwilling victim must be grappled before the succubus can use this ability
As has been previously stated, Pathfinder distinguishes between grappler/grapplee, but insofar as I can tell, not in a way that clearly enables us to determine whether or not a grappler is "drainable."
However, if we read it in a certain way, it is plausible that the caveat for unwilling victim is that it must be grappled in order to be forced into an act of passion. In other words, the act of grappling with a succubus is not passionate, but during the grapple the succubus could force you into an act considered passionate- or rather into an act which would only be willingly performed as a result of love or lust.
This is, of course, hypothetical supposition. In the interest of true scientific analysis, we'll need to gather empirical data concerning the nature of grappling positions which could be construed as forced acts of passion. We will then determine whether or not these qualify the succubus to drain energy, and if these positions do so to the exclusion of positions not considered passionate.
In the interest of PURE SCIENCE I will volunteer to unwillingly roll in the proverbial hay.
Also, 800+ posts and nobody referenced Nale and Sabine? Really?