When I'm having sex with someone else it's all big O's. But when I masturbate there's two kinds of orgasms I have. One is just an end to the fun; there's a very minor crescendo and then I mostly know that I've come just because I don't feel like masturbating any more. That's probably 90% of the time.
The other 10% is an orgasm so big and powerful that it's downright embarrassing, so big I won't let myself have it unless I'm alone in the house because of all the noise. If I do go through with it I know I'm going to lose control--of my hips pounding so hard the bed shakes, of my voice moaning out loud, and of my pussy just coming and coming and coming.
Man, if I were a Martian I wouldn't understand at all what people find embarrassing. If I cut myself I have no problem screaming "OW" and letting everyone know how I'm feeling, but I'd rather suppress an orgasm than risk anyone knowing I'm feeling good. It's only polite.