If you look at the inner mouth, you can tell it's probably a tongue, because in the scenes you can see it open, there's no tubing inside, it's just flat behind the teeth, I think they're merely used as a kind of spiked battering-ram attachment so the rod does more damage when it is launched towards the prey.
And it has always been kind of penis-like, in a way. The scene in alien where it is thrusting in and out of the mouth with whitish goop dripping from it, if you understand the mechanics of intercourse, you'll understand why this likens it to a penis.
As for the point about it becoming seemingly erectile when killing, I agree, though I'm not too sure it's sexual pleasure it gets from killing, but its an instinctual thing.