The number of jokes about family trees that have no branches attribute to what many people believe about the 'disgust' of incest. If, for a moment, we put that disgust aside, let's ask what is wrong with being physically intimate with close relations? If we grew up with them, why not procreate with them? Isn't it easier to build a relationship with someone you've known a long time and you know a lot about them? Look at benefits of similar backgrounds, similar believe systems, similar culture, etc. So why not?
Well, the first "don't" we'll discuss comes from the medical field: genes. It appears that none of us humans have a perfect set of perfect genes or even of a perfect D N A pattern. All of us have flawed cells that make us who we are. However, good cells when matched with bad ones usually win out, though not always. The more closely related to us someone is, the closer these cells match, and the further apart, then the less-likely we are to having the same pattern. I got several bad genes from my dad, I'm sure, but my mother had good ones in that same area so that balance kept my gene nearer to the normal range. If my parents had had the same bad genes, then I would have had greater defects in my physical makeup. To put it more bluntly, the chances of having a deformed, or malformed child go up when the parents' gene pool is more similar.
Since, at least in America today, a discussion of what 'sex' means, let me touch upon that. Once upon a time, what we call sex was called 'sexual intercourse'. It was intercourse of a sexual nature. Now some use the word intercourse for sex, but that is not accurate. Sexual intercourse is 'having sex', but intercourse between people is sharing, discussing, debating, being open. You may say that having intercourse with someone merely means that you are sharing each other with each other and learning about each other. Like any relationship of depth. So, permit me to say that 'spiritual intercourse' is intercourse on a spiritual level.
Taking that medical lesson to the spiritual level, gives us the following: If the only 'spiritual intercourse' you have is with people that believe what you believe, then you are in trouble. Each of you has the same set of 'bad spiritual genes' and can not see the blind spots in each others belief system. You will only magnify your spiritual limitations by surrounding yourself with 'spiritual yes-men and yes-women'. If you want quality spiritual intercourse, and quality spiritual surroundings, I believe you MUST have it without having 'spiritual incest'. Get discussions (not debates, not fights trying to out do each other) and conversations going with people that disagree with your beliefs.
Just as physical incest has a greater chance of producing mal-formed children, so does spiritual incest have a greater chance of producing belief systems that are mal-formed. If you want the truth, and don't want to be diverted from it by your own flaws, then find someone without your belief system. Catholics, Baptists, Friends, Jehovah's Witnesses, Jews, Moslems, Buddhists etc. would all benefit from tolerating each other and having spiritual intercourse together.
Whatever 'faulty genes' their current belief system has, they reinforce by having 'spiritual intercourse' with their own kind!
This is also true of those with various political and social and economic beliefs! If you are a liberal, and only spend time with liberals, you'll never understand some truths about the limits of your beliefs. Ditto for conservatives. And libertarians. And socialists. EVERYONE!
Hey, guess what. You and I probably just had a touch of spiritual intercourse as you read this! Odds are high, that you don't agree with me on this issue 100%. Please, let's continue our spiritual intercourse. I believe we will all benefit from it.