NOTE : already posted that in another form earlier here (and on /r/MGTOW) but it was removed, so here's a new version with the offending parts removed, and made more legible by emphasizing the keywords, this version has no association with /r/MGTOW or associated concepts

First off:

There are days in woman's cycle where plan B has 0% effectiveness.

Here's a great article, aptly titled Everything You’ve Been Told About “Plan B” Is Wrong.

Now I'll just clear misconceptions I've had to explain to too many guys. The probabilities are approximative, and the vocabulary not clinical at all, to keep it simple (and also to not have to look up every number, but I'll provide links to places where you'll find them).

This is focused on contraception only, not STD prevention, if you want to avoid STDs make sure that every sexual contact is protected with a properly used male of female condom, or a dental dam

As a guy I'd say spermicide + male condom is very good : cheap, can be managed all by yourself, and easy to carry on your person (make sure it's your own condoms, and you can put them on right, because any mistake on your part will drop the effectiveness from about 99% to 75%). Female condoms can be good too, make sure to bring your own (but I've yet to see a woman who prefers these to the male version). Carry both, they're cheap. The STD protection aspect is of course a nice perk. Only issue with the spermicide is that it has to be inserted in advance (it's more of a female contraception that way), but if you ever have a spontaneous sexual relationship where waiting a few minutes is not an option, using only a condom (properly) should still keep you safe enough. Don't be lazy : be the one to dispose or your own condoms after sex, this way you should be able to notice tears right away. This is why it's (IMO) the best option for men, because you'll notice anything going wrong right away.

If you're very adamant about remaining childfree : don't trust anything you can't check by yourself. So a woman telling you she's on the pill/implant/injection, or has an IUD, is no guarantee. A woman telling you she's using a diaphragm, sponge, cervical cap... might either not be using one, or just using it wrong. Pills can be forgotten. Injection or implant might be older than originally thought (and not providing any protection anymore). IUDs are safer, but if she forgot to get the necessary check ups, it might not be in working condition anymore. Unlike a condom, you won't be able to check it out yourself.

The planned parenthood website is great for learning more, but a lot of the info will not be very useful to a single guy who wants to be able count only on himself (and not rely on trusting anyone else). Of course the best case scenario is two people having sex who'll both have their own ways to deal with contraception, but I wouldn't want to entrust my future on someone else's decisions.

Spermicide (if used alone) would work marginally (reduce the risk of pregnancy, but there would still be significant danger).

Pulling out, if done right, can b surprisingly effective (~96% success) but still a risk. Other methods considered effective (if used correctly) have at least a 98.5% effectiveness. So even when done right it's still 3 to 4 times worse than what we'd consider "effective". And if done wrong, it's the same as unprotected sex.

Spermicide is a coin toss (about cuts the chance of pregnancy by half but that's it). So it's only useful as an (imperfect) second line of defense, which can be combined with any other method (unless maybe sponges, but only because they already contain spermicide).

Then there's the "holy grail" : vasectomy. But even that has to be done right. Remember to check your fertily yearly after the procedure (might do it less often after some time) because it might revert by itself. Also even with vasectomy and regular tests, there's no 100% effective method. Even if your tests are good and you're still "infertile", you'd just need to have one sperm at the wrong place at the wrong time, or for your vasectomy to naturally revert between tests to make someone pregnant. So even if it's kind of a "lightning hitting you twice" kind of chance, know the possibility is there.

This section is for female readers and men in long term relationships where trust isn't an issue :

Hormonal contraception methods (pill/implant/injection, and to some extent hormonal IUDs) can be defeated by some health issues, or interaction with medication. Here are a few examples even medical doctors forget to tell patients : it doesn't work reliably on women above a certain weight, antibiotics can make them useless, some hormonal issues might also have an effect... So in case some women are reading my post here's a good link about a long list of stuff to watch for, and remember throughout your life to check for new information on that subject (every few years seems there's something new that is found out).

Some rythm methods (those that require to track 2 or more physical signs) can be very effective (some boast 99.6% success rate in clinical studies, as long as you avoid unprotected PIV sex during a specified window in each cycle). But those require a lot of trust in your partner. If used wrong, these are just as bad as using nothing at all. Honest mistakes happen. So these should mostly be used as complements to another method (you're going to have to go protected around ovulation anyway).

In any case always try to combine two or more methods. Best case scenario are two partners each using their own methods of contraception. You could end up combining up to 4 methods which gives you great odds. But even as a male relying only on what you can use and manage yourself, you should be fine using male of female condoms + spermicide.

In the end you might as well avoid vaginal intercourse altogether if you psychologically can't accept the fact that no contraception method, or combination thereof is perfect (even though combining two effective methods will give you astronomical odds of not getting pregnant). Whatever happens, it just takes one time for a guy to lose all possibility of being childfree (or at least for that choice to belong to someone else).