I'm no expert on this, but I'll share a few thoughts. To a large extent,
people hang out with people like themselves. The more they have in common,
the more likely they are to socialize. So it's natural for Christians to
socialize with Christians, Wiccans with Wiccans, etc.
That said, I'll try to address the "only" part of your question. Christ
admonished his followers to a maintain a degree of separateness from
"the world" and "those in the world". Many Christian churches today do
the same, though they take it to varying levels. For example, I was raised
as one of Jehovah's Witnesses. JW's are quite strict about socializing
only with other JW's. This is spelled out quite clearly in their
publications. Even their vernacular illustrates their separateness.
They refer to other JW's collectively as "the friends". They describe
other JW's as being "in the truth". In contrast, anyone who is not a JW
is described as "worldly" and being "in the world". Even though JW's are
generally quite friendly, in their church, making friends with non-JW's is
strongly discouraged because "bad associations spoil useful habits", etc.
I suppose from a memetic perspective this is a great defense mechanism.
If the JW meme complex succeeds in getting a person to associate closely
only with other persons infected by the JW meme complex, then there is
much less chance of that person getting infected by memes that would
cause them to reject all or part of the JW meme complex and leave the church.
At some point I would recommend asking this girl if she would be willing
to be your friend even if you do not attend her church or accept her
religion.
Joel