"You would marry someone else."
Translation:Vzala by sis někoho jiného.
You would certainly be understood and many natives would use it without suspecting a problem.
In articles like http://nase-rec.ujc.cas.cz/archiv.php?art=7171 you would find long discussions about the views of different scholars in different times in the past. The article also brings examples of this form from several respected authors of the late 19th century and the first half of the 20th century.
Here we generally follow the codified grammar of standard Czech as described by the Handbook of the Czech Language Institute of the Czech Academy of Sciences. This handbooks simply says "Ve spojení se zvratným slovesem jsou ve 2. os. j. č. spisovné tvary: koupil by sis, umyl by ses." I.e., "In connection with a reflexive verb the standard forms of the 2. person of singular are: koupil by sis, umyl by ses."
The main point is that the two s in sequence are hard to pronounce.
I used "vzal" instead of "vzala" this time, and it was accepted, so "someone else" seems to be non-gendered. Am I right about that? Thanks!