This is your autonomic nervous system.
One of those pernicious misconceptions about biology is that “you” — the conscious, aware, self-reflective part of your being that thinks and plans and takes in sensory information and initiates voluntary movements — is the whole of your being. It isn’t. It’s a small fraction of the activity of your nervous system. It’s a layer on top of a whole hierarchy of control that works away without “you” thinking about it. There is this whole parallel chain of distributed ganglia in the autonomic and enteric nervous system that are pretty much autonomous — they don’t take orders from your conscious brain, and they don’t report back directly to “you”, they just do their job. It’s hot, you don’t have to instruct your sweat glands to start secreting. Also, you’re nervous — likewise, your sweat glands are getting autonomic orders to get to work.
You don’t have to think about peristaltic rippling of your colon when you use the bathroom; those smooth muscles are coordinated by ganglia outside of your brain. You don’t send explicit commands to your irises to dilate when the room lights dim — there are circuits in your peripheral nervous system that just do it.
Also, fortunately, when you’re getting romantic, you don’t need to control the minutia of your body — you don’t have to consciously transmit signals to your genitals to “Vasodilate…now! Prepare to secret lubricating fluid! Engage veinous valves! Potentiate sensory input!” It’s all done for you by unconscious regulators, a set of autopilots to control the details at the cellular and tissue levels.
Unfortunately, these circuits can also operate without willing cognitive direction — they’re local and stupid and just drive the machinery. That’s why you should read Ally Fogg’s post on rape myths — both men and women can be betrayed by low-level autonomic responses that are not acts of will or desire.