Every single study (both saying sperm is present and those saying it's not there) have issues and none of the studies are able to be replicated (which is a cornerstone of the scientific method). Some issues with the studies saying no sperm is present is that they didn't check sperm count within two minutes (so they started dying), they didn't ensure urination between samples and most importantly small sample sizes (you can't accurately extrapolate the data for the general population). As for the studies saying sperm is present they too had issues like small sample sizes (same as above), there were concerns that some of the men didn't produce precum (or used regular ejaculate in its place) and it goes on and on though they did have participants urinate, and checked with two minutes. What they need to do is run a proper clinical trial with a significant cross section of ages, races etc and ensure the participants urinate before a sample is given. Until these criteria are met I think it would be safer to err on the side of caution and treat precum the same as regular ejaculate especially if you're trying to avoid pregnancy. In my opinion, it's not worth the risk to possibly get pregnant because of faulty science. If you're within 72 hours from the time you've had sex and don't want to be faced with being pregnant (and deciding what to do from there) take a plan b. If it's past 72 hours all you can do is test if you're late