Most Helpful Guy
If his female relatives are immature enough to prejudge you and consequently dislike you, then perhaps they are immature enough to say or do something insulting to you, in which case your boyfriend may be keeping you away from them to keep the peace.
Most guys consider themselves fixers/problem solvers. With that said, guys don't like to get involved in women disputes, etc. if we personally can't resolve them. Perhaps your guy knows that the problem isn't fixable.
A proper introduction is the traditional, respectful thing to do; however, that only applies to ideal situations.