Maybe I misread you first sentence. But as a Christian myself, I get irritated when people who are not Christian's asked loaded rhetorical questions like "how can people be Christians, its blah, blah blah." Some people don't understand, I get it, it is their choice. By the same token, I don't understand atheism either, but I don't ask "how can they be atheist," either.
Some Christians and atheist alike will tell you that the swinging and Christianity are incompatible, you can be one or the other, but not both. We have found a way to be both and are comfortable with it.
The same goes for beliefs with Christianity. There are many traditions, rules and "laws" we don't agree with or apply to our lives. Sure there are basic tenets that are the pillars of Christianity, but many of the things in organized religions are constructs that we feel don't apply. But as long as they are not harmful to others we believe in live and let live. If some Christians believe a robust sex life is not part of Christianity, then so be it, too bad for them.
So how can anyone be a Christian and believe or not believe something? We each choose our own path, we respect peoples right to choose their path and hope for the same from them. If they are genuinely curious to know how we came to those decisions we will try to tell them.