Well, before the changes from some of the last few updates the pressure against mixing races was explained. Though, they then re-did some of the characters (which I find great I think the stories and sprites are better now). Anyway, assuming the reason against mixing races has not changed, it is that even very closely related species mixing will most likely result in children with extreme birth defects resulting in their death. On Maccon's old route, he explains that his (I believe) cousin died at the age of five because he was a mixed species. His organs were so malformed that he was bed ridden until he died. They even mention that outside of the states (I am assuming the United States when they said that) that mixed species marriage is not allowed specifically to prevent that. Maccon then asks the Main Character if he has any defects, the main character says no that doctor's have exaimened him before and that he seems to have only inherited the stripes. Maccon then says he is happy he is healthy. Being mixed is also the reason Jayce, Main Character, hides his stripes under a hat as his father was at least part tiger. His hat being taken and Maccon seeing his stripes being the reason the whole conversation started.