Where I live the holiday isn’t called just Valentine’s Day, it’s Saint Valentine’s Day and since the saint was male,
Valentine is male to me (also it’s Sveti
Valentin which is obviously male, since the female form would be
Sveta Valentina).
Regardless, I prefer
Valentin for a boy and
Valentina for a girl (the same way I prefer
Augustin and
Augustina to
Augustine).