Your rationale for not getting your Yorkie neutered is that "he's never been aggressive towards anything or anyone!" and the opinion of your BF.
No, I have not done extensive research on the topic, nor do I represent a statistically significant sample size, but you said all opinions are welcome so here is my experience after having owned 2 males and 1 female for many years... My current male who is a very sweet dog when he gets to know you is also aggressive at first encounter; he will nip at new people when they come in our house or on walks if I were to let him. He was neutered like my other one who was not aggressive to strangers at all. There appears to be no causal relationship per my experience, between neutering your pet and its aggression. Now is that scientific? No, but it is the truth with my 2 little males. I can also say that humping has not been a problem with my guys since they were little pups, even though it is supposed to be a sign of dominance and does not necessarily have anything to do with mating. That being said, there is some risk of a mess, or so I have heard, if you don't neuter, and of course the medical issues the vet will tell you about. I would be interesting to hear from owners (not breeders) that did not neuter and their experiences with that. If you never let your Yorkie out by itself not on a harness, which is the case for me, I think not neutering might be not be a bad thing, but you’d have to weight the risk of testicular cancer (and I don’t pretend to know what that is). I think not neutering would be more risky for some owners around my neighborhood, who let their little dogs out in the yard apparently unsupervised.