Ahaliyan e kutub (jews, christians) are people who believe in the holy scriptures before Quran, as well as the messengers who conveyed those scriptures.
People who believe in Quran and its messenger Hazrat Muhammad (pbuh) are called muslims. Believing in him automatically means belief in completion of the line of prophet hood or any synonymous terminologies e.g. manifestation, messenger, etc. None, thereafter.
Muslim men may marry ehl e kitab women. Muslim women may only marry muslim men, for reasons including but not limited to lineage and inheritance. Neither may marry non believers which includes atheists as well as followers of all religions/faiths/belief systems except the ones mentioned above.
Rules are rules.
Having said that, don't give a rat's ass what anyone does in their private life. As long as in 25 years some fusion critters with gulabi urdu and mile long churned up surnames like Bhutto-Zardari-chaudhary don't come chanting to Pakistan, promising to give roti kapra aur makan to our next generations. Who, sadly, might still be lacking the said commodities in their lives. Because the critters were being raised on their money all the while. As usual.