If a kid asks me of sexual stuff, i'll think twice, because what I say can be transmitted to parents in a different shape.
Let me ask: will you answer "ask your parents" in the same situations? About the religion - do you consciously try to avoid confronting a kid with your beliefs (that may be different)? (just want to figure out)
For the most part, what I tell my son is the truth, though surrounded what I consider to be an appropriate perspective.[1] I think moral authority is important as a parent, and I think that is more solidly gained by telling your child the truth, so that they know they can ask a question of you and you'll give them a straight, true answer, as best you can. Lying about your own foibles, as PG points out, is a "cheap" method of gaining moral authority that backfires badly.
[1] I think a lot of "lies" we tell children have to do with that surrounding perspective. When we lie to avoid discussing certain subjects, it's because a child is not yet equipped to understand enough of the surrounding perspective to make it worth trying to understand the "truth".