Sperm is virtually entirely absent from pre-ejaculation fluids if you've peed since the last time you ejaculated. Almost all of the "sperm can be in pre-ejaculate" effect is from having sex a second time in a row without anything flushing out the tubes.

Fair, and it needs clarification to avoid conflating pre-ejaculate, seminal plasma, and sperm emission.

It may be confusing, so to clarify: "seminal fluids" is a term typically used to refer to the fluid released during ejaculation, not throughout the arousal phase. The idea that sperm would be in the mix before the emission phase goes against standard reproductive physiology.

Sperm are only actively introduced into seminal fluid during the emission phase of ejaculation; the so-called "grand finale." :D. Before that, in the arousal phase, the fluids released (like pre-ejaculate) typically contain no sperm unless there's residual contamination from a previous ejaculation.

It's not. I think johnisgood and loeg both know this, but they're being dangerously simplistic in some of their replies.

If you recognize emission — not just when expulsion is imminent — and if you pull out and that's the end of vaginal intercourse until you've cleared the urethra again, then that's probably nearly perfect at preventing pregnancy.