Well, true, though the vagina is considerably closer to the anus than the mouth is.
Also, never underestimate how clumsy or inaccurate a sexually-aroused person in a darkened room can be.
Possibly urban legend, but Dr. Ruth Westheimer, when asked how many sperm it took to cause a pregnancy, allegedly said, “just one quick one.”
I would prefer methods in which user error isn’t a factor. The most a appealing are hormonal IUDs, copper IUDs, and a Nexplanon implant. Depo shots are also an option but I’d prefer something removable in case of adverse effect from hormonal BC.
I tried reading some technical papers about these methods but I’m not sure I fully understand them. They also make it sound like we don’t really know what percentage of times the BC prevented fertilization rather than preventing implantation.
I assumed copper was the best because it stopped sperm. But some of what I saw also suggests it ( rarely ) also stops implantation.
Which then makes me wonder - are hormonal methods more effective at preventing fertilization because they often prevent ovulation and egg availability whereas copper does not? Or is copper so much better at stopping sperm that it doesn’t matter if ovulation occurs? I haven’t found anything yet that conclusively answers this question.
Between a copper IUD, hormonal IUD, or Nexplanon implant, which is most likely to prevent fertilization?
Separate question: do hormonal IUDs have fewer side effects than birth control pills since they’re delivered to the right area more specifically and presumably require a smaller dosage?
The mode of action of IUDs - PubMed it’s a study that seems to address this question although it’s 30 years old and I’m not sure I fully understand the conclusion.
I was thinking the same thing.
As I mentioned to a friend of mine whose wife was regretting his vasectomy choice - “not to worry, they can harvest sperm. All they have to do is stick a big, long needle into your testes.” 
Birth control pills do allegedly prevent ovulation form happening at all. Of course, like all fire-and-forget mechanisms, nothing is 100%. Condoms leak, IUD’s pop out, pills don’t work 100% sometimes, foams and creams get sloshed out, diaphragms shift, the doctor “misses” or the tubes grow back together after a vasectomy or tubal ligation, etc. The only thing guaranteed 100% is either abstinence, menopause, or removal of the ovaries or testes - ie. the bits don’t do what they should.
Isn’t she the one who said that if you’re using the rhythm method, the time to have sex was at the mid-point between periods?
With sperm, it’s a group effort - the first ones out of the gate break the enzymatic lining of the egg, lose energy to continue so they cannot impregnate, but if there’s more sperms swimming in the back, one of them will find the eroded area of the egg and get in. This is why a high sperm count is helpful in impregnating someone. It’s helpful to know also, the little fellas can live up to five days. A careless man would indulge five days prior to someone’s ovulation and ovulation is not really certain all the time. This is why people using the rhythm method for birth control are called parents. With all of the certainty we know about sperm, snipping vas deferans is the way to go.
I don’t know the answer to any of your questions, but I wanted to make it even more complicated 
Some contraceptives (I don’t remember which) work mostly by preventing implantation. Ovulation works, sperm swim, fertilisation works – and implantation fails.