I reckon it probably isn't common to get pre-cum without an erection. It has never happened to me but then I don't get a lot of pre-cum anyway and have to have been aroused for a while to see any appear at the tip of my penis. But people are different and I know someone who produces more of it and for whom it appears much earlier in being aroused.
The glands that make pre-cum, the bulbo-urethral glands, are the male version of the glands that make vaginal lube in a girl (called a homologue) so they are probably linked to getting aroused by the same mechanism.
Then, there is nothing in the whole process of getting aroused and even cumming that requires an erection, it's just that an erection usually happens quite early in the arousal process but, if something else stops the erection, for example being drunk, that doesn't necessarily stop everything else.
The glands that make pre-cum, the bulbo-urethral glands, are the male version of the glands that make vaginal lube in a girl (called a homologue) so they are probably linked to getting aroused by the same mechanism.
Then, there is nothing in the whole process of getting aroused and even cumming that requires an erection, it's just that an erection usually happens quite early in the arousal process but, if something else stops the erection, for example being drunk, that doesn't necessarily stop everything else.