Though I get why. Umbreon is Dark-type (Evil-type in Japan), which is sexy. Sylveon is SUPER feminine-looking, which lends itself very nicely to either being a straight up female or the ever-popular “trap/femboy” fetish (especially since, what, 3/4ths of all Sylveon are male?)
Supposedly in X and Y’s case in the Japanese version, “X” and “Y” are just nicknames and their full names are Xavier and Yvonne (much like the names used for them in several pre-release screenshots of the games). But for whatever reason, the English translation opted to completely ignore this.