I've never ever heard the phrase "we're pregnant" used, so I don't have a prior opinion on it. Of course, reading how people feel about it I am convinced it is inconsiderate, but I wished to add some general thoughts about similar phrases.
I've heard a lot of people say "we're going to college" when their child or grandchild is going to college, "we're sick and can't leave our bed" when their child or grandchild is sick, as well as "we're becoming fathers (mothers)" when their son (daughter) or grandson (granddaughter) is having a child. I always took that as a deliberately silly attempt of involvement, and thought that nobody hearing a similar phrase would have doubt in their mind who is working hard to get into college, who is fighting the sickness, or who is the one who will have the child – besides the parents and grandparents have fought harder challenges in their life, so it is not like they are belittling anything. I think that hearing your mother say "we're growing a beard" about her grandchild is funny and endearing, and in that context it is a metaphor I am fond of for just how silly it is.
On the other hand, the topic also made me think of how easily people who are not soldiers say "we're fighting a war", and people who are paying someone paint their house and install their furniture say "we're painting our house and installing new furniture". Dismissing someone else's hard work seems to be a huge thing with us people. In these cases the person doing the work has been practically forgotten, and I think that it sucks that our language has evolved to a place where we're simply not giving the right people the credit, and in some of them we're forgetting to do so every damn time. That's not only inconsiderate, it might even be harmful. I wonder what the change of perception would be if never ascribed the fighting in war to ourselves.
Probably without being the right person to speak about it, I think "we're pregnant" falls into both categories. An over-excited man claiming to be pregnant-by-proxy when they are ignorant of what that entails is funny. But I don't we should be taking the credit for what someone else is doing, let alone making a habit out of that.