GiantSteps I find your post uncomfortable reading.
Why would a transman want to get pregnant? Well, I would imagine for much the same reason you or I would want to get pregnant - to have a child. Also, people's feelings change as time goes by. I would hazard a guess that 18 years olds having their breasts removed may not feel BFing is at the top of their list of priorities. As time moves on their feelings on that may change.
How would they do it? Well that's their business - I don't go about asking my lesbian friends with kids how they managed it unless they volunteer the information. But to assume that they wouldn't be having PIV sex unless possibly rape sounds incredibly offensive to me!
Transmen aren't one homogenous mass. Like any other groups of people it's a gourp made up of individuals with their own thoughts, feelings and sexualities.
This as well "If getting pegnant triggers dysphoria, then surely as well as a mastectomy transmen might be looking for a hysterectomy or tubal ligation?"
You're putting transmen into one box and trying to work out what all of them must feel using logic about a situation you know not much about. If you're curious, why don't you do some research online to find out more about transmen and pregnancy instead of making sweeping and offensive statements about them?
Your post smacks of "who would want to have sex with them?!" which is I strongly suspect not what you're trying to say.
As I said upthread, I'm deeply troubled about the rise in trans ideology and I'm particularly worried about the rise in women persuing transition as a route out of the gender roles that our society is imposing on us.
If we are to support our daughters growing up in a culture that tells them they need to chop their breasts off in order to reach acceptance then - as well as fighting that ideology with all we have - we need to treat those who have chosen that route with respect and compassion, and not other them like this and treat them like freaks. It's very dangerous IMO.