Using mRNA as the vaccine vector is similar to how HIV works (HIV is a lentivirus). Per wikipedia "Lentiviruses can become endogenous (ERV), integrating their genome into the host germline genome, so that the virus is henceforth inherited by the host's descendants." Basically if an mRNA vaccine is messed up, it messes up your genes and any offspring's genes.
Source for parallels between mRNA vector and lentiviruses
https://www.tandfonline.com/doi/abs/10.3109/10428194.2011.634048