(With apologies to Elizabeth).
The feeling of sexual excitement is a good one, even if you're not getting any sexual gratification. It can be on the sly at work, or even just at home - in fact, you can be sexually aroused anywhere, providing your mind is inclined that way. I remember letting my thoughts wander and suddenly feeling horny halfway through watching Harry Potter and the Chamber of Secrets. But I digress.
I guess it must be different for girls. For a boy, if you feel horny, you eventually get an erection. It's not instant (as the rumourmongers would have you believe), but you'll get an erection, and soon, after the first moments of feeling turned on. Maybe a willing girl will explain how you feel when turned on in a public place?
Anyway, there's that jolt in your stomach you get just when you:
(i) realise you have a crush
(ii) see someone you think is attractive
(iii) begin to feel horny
It's not the same jolt. Just before I get a real craving for it, my stomach radiates a dull, pleasant sensation, which is different from the nervous, sharp flip you feel during situation one. Nevertheless, it's pleasant, but a little guilty; every now and again, I think to myself something like, "I might masturbate when I get home," or "I had sex last night, wasn't that great?" or even "hey, I have that new soft porn DVD I haven't watched yet" (even though I haven't bought one of those for ages...), and before anything else, there are butterflies starting to stir.
I don't know why. It's got to be a psychosomatic thing. Your vision might go blurry before a migraine - a migraine aura - so maybe this is a "sexual excitement aura"? Something which says to me, "you are about to be hard". (Or, "you are about to be wet"? Sexual equality, and all!) And it lasts anything from a few seconds to however-long-you-can-go before surrendering and actually becoming sexually aroused (like during my A-Level History exam... stomach felt great, but had to keep bringing myself back to World War II, which wasn't as appealing as Lisa Boyle in Friend of the Family just then...).
It's often scooted over in favour of portraying people in a heightened state of sexual arousal. But there are those vital moments just before you're excited, those moments when you know you're about to be...
...and isn't that an exciting prospect?