When you think about it, Christianity has some fucked up relationships.
To begin with, it is an one parent family. There is God, there is the Son of God (while they still pretend to be monotheistic) but there is no Goddess.
Now, Mary someone could say is the mother, but she is not exactly referred as being of equal status, neither she is exactly thought as "married" with God. So essentially God has a child out of marriage.
Worse, Mary was already married, (to Joseph grasshopper) so God had a child, with someone else's wife.
However, in any case, we are all children of God in the first case. So that makes it incest.
Or in fact, makes it a relationship between a grand-grand-...-grand-parent and a grand-grand-...-grand-child, which has a certain iffy factor of its own.
Finally, if the gospel of James counts, according to it she got married to Joseph at 12, and got pregnant by God at 16. (References: catholic library grasshopper:
http://www.catholic.com/library/mary_ever_virgin.asp + Protoevangelium of James 8 & 15). Which makes God a... weeell, let's just say that the age of consent in Texas is 17.