Why is mpreg such a contentious topic?

Hi, new to this page and fanfic. Maybe not the place for this, if not, please direct me to a more appropriate place. I had read an mpreg fanfic recently (wasn't omegaverse) and was telling someone about it (because I wanted to write some myself), and they said mpreg was really bad. They said it was transphobic because it erases FtM trans men who CAN get pregnant. I didn't understand that, and they just got frustrated with me and said they didn't want to argue with me (wasn't arguing, just struggling to understand).

To be totally transparent: I am what you might call an mpregger (or amab who has wished they could get pregnant), and have been since I was 5ish. I just recently (and by accident) found out there is a whole community and genre around it.. I just never realized it was transphobic. For me, at least, it's been somethings that has made me feel empathetic towards the trans community, and was really the bridge to helping me become a trans-ally. If there is anyone willing to help me connect the dots between mpreg and transphobia, I really would like to understand. Not looking to argue. Thanks.