I think a lot of it has to do with the fact that in a work of fiction, people only get pregnant when it's convenient for the plot. Someone who's had as much sex with as many different people as Bull has would have almost certainly knocked one of them up by now.
As for what happens under the qun, I'm not sure, beyond that I assume they take specific contraceptive measures for the "cork popping" the tamassrans do.
Um yeah pretty much agree with you here lol. Unless all those women were using protection, not one got pregnant? Which like you mentioned, either:
1) The story doesn't require of any half-Qunari bastards walking around or,
2) It's just impossible to procreate with a Qunari (Ox-man).