I thought there had already been such a thing a while ago, and the men did not object to the needle, but complained about the side effects (which were the same as those for the pill, and the bulk of it was only imagined by the men, i.e. the placebo group complained, too)
0nline, I do not really believe this claim that men do not choose to have children. It is like this rumours that women are the ones who want to get married. In fact, abusive men very much like to marry women and get them pregnant; this is a known fact.
Decent men quite likely are not the driving force behind family planning because pestering a woman to endanger her health and wellbeing to gestate some babies would be a very shitty thing to do, and if you're not Henry VIII, you'd be reluctant to do that.
Doesn't mean they'd be too lazy to remove a contraception decive if they were offered the opportunity to become fathers.
I suspect that most men who "accidentally" become fathers are in fact very happy to spread their genes (there's this ego factor about it, wasn't there this guy who worked at a fertility clinic indirectly raped dozens of customers by impregnating them with his sperm instead of their husbands'?), but would like to avoid the responsibility that tends to come with it. Which they do by not using condoms / not using condoms correctly, and then acting all surprised if a pregnancy results.
Nevertheless, it would of course be interesting to observe what happens when the "I hate condoms" and "vasectomy is too final" excuses were taken away.
I do not think there will be a significant decrease in the number of wanted pregnancies; or even overall pregnancies. I'd guess that the men who previously used condoms because they're no fucking idiots would get the jab and be happy about being able to not use condoms with their long-term partner ... while the irresponsible idiots who didn't use condoms before will not get the jab, either, and quite likely still complain if they become fathers "unexpectedly".
It would just all be a bit more obvious to the onlooker what is going on.