Can a Sunni Muslim marry a Christian?
Of course they can but I am sure they must make a few compromises in their lives.
Per Islam religion rules, a Muslim man (Sunni or not Sunni) can marry a Christian woman (Catholic, Orthodox, or other denomination). However, children should be raised on Islam teachings until they are grown up and reaching Puberty such that they can exercise there full free choice.
According to Islamic Law, only a Muslim male can marry a Christian female. No other combination is permissible (i.e. a Muslim female with a Christian male, a Muslim male with a Christian male, or a Muslim female with a Christian female). According to Christian Tradition, a Christian should not marry a non-Christian, but it is permissible. According to Western Law, the religion of two partners seeking marriage is irrelevant to the State.
Are mixed marriages accepted or rare in Pakistan what are the chances of a Muslim guy marrying a christian girl?
No, A Muslim girl cannot marry a Non-Muslim man, may he be of any religion. Only a Muslim man is allowed to marry a Jew or a Christian woman provided she is a true believer in the ONENESS of Allah Almighty. Most of the Christian women don't fall into this category. A Jain boy cannot marry a Muslim girl.