We did have pre-cana, actually twice, before we got married (one by the priest who married us and one with the necomb center at her college...our priest required us to use his). Sex was discussed, but only in a more or less cursory manner, and she pretty well tuned it out (that should have been a warning sign then, but I didn't recognize it). She says it ins not a religious thing, just that she feels like she is intruding and doesn't belong there. The pregnancies didn't seem to bother her much (except the well meaning folks that have asked her when she's due 6 months after the baby is born and those who react like she's ready to drop the baby when she was only 7 months along (she carries big). She did pretty much avoid the birthing material in the birthing classes, which we only did the first time around.
It seems to be mostly a notion that 'nice' people don't talk about sex that was apparently the attitude around her FOO.
