To me, it's just like a taboo. Like it's disgusting for me to think of my siblings. But I think it's also to the parents. For their kids who came from their own blood to have an incestuous relationship is kinda...not pleasant. But most people just grow up thinking it's a taboo and it passes on generation.
I have like 6 other siblings, and its like something sacred, u know? I can nver imagine it on my family cuz its something precious to just "sexually touch"? Hard to explain tbh but my opinion ┑(￣Д ￣)┍
Well, first off any sexual taboo including what constitutes as incest is socially constructed. But there are a lot of practical reasons for it. Sex has a way of messing relations up and being a family is hard enough as it is. In the past when marriage was more about alliance and land, at least for nobles cousins were often married. And in villages everyone was related to everyone. Strangely, sex and marriage have long been two different things, since many people had sex but couldn't marry (yet). So there was always a line drawn as to whom one could "mess" with.
Personally, for example, I had strict rule of not messing about with friends, classmates, dormmates, and later co-workers unless we took the time to get emotionally serious first. So far I haven't had to worry about it, but messing about with someone that you then have to see everyday, or lose a friend over isn't worth it. So you could say in a way I had my own taboos.
Well first off its unhealthy literally, if we are speaking of a hetero couple. Any child born will have defects, inbreeding is not a good thing to do.
In general our nature as mammals dictate for us to find most suitable mates to our own blood. When say siblings who haven't been raised together but share same blood will met they will feel a very big physical attraction to each other. Growing up together and being part of a society constricts the sexual aspect of love. Of course there are societies out there who aren't against sibling love. People from that area won't find anything wrong with it.
For us and most of modern world concept of morality sexual desire between siblings is not acceptable. Partly cuz of whole taboo thing but also cuz our nature is to bring forth healthy strong offsprings.
Like Vulshi said, it really comes down to problems with inbreeding - birth defects, inherited diseases, etc., but of course only a male/female incestuous relationship would result in pregnancy.
Morally, I guess everyone has their own opinions. It obviously wasn't an issue in the bible, for example when Lot's daughters get him drunk in order to sleep with him to get pregnant, because after the destruction of Sodom & Gomorah their husbands were killed and there were no more remaining men. People were too stupid back then to know that incest caused birth defects, and apparently God didn't have an issue with it anyway.
That's why I think incest between brothers or even a twin brothers are the holy grail of this taboo.
i can see why incest in general is condemned for three reasons
-it's often an older family member taking advantage of a younger family member out of lust
-it's much more likely to result in unhealthy offspring
-it contorts family dynamics
The reason I have heard why it's bad is because regular family relationships tend to be in a hierarchy (without sex involved). This means if you have incest relationship often one member has more power over the other. This power can't be cut because the underlying relationship of family can't be changed. Obviously some exceptions do apply, but for the most part a parent has power over a child and elder siblings tend to have power over younger ones. One person having more power in a relationship is always bad and makes an unstable relationship. Relationships tend to work best when each party sees the other as an equal.
So basically what I'm trying to say is that most of the time an incestuous relationship is not going to be a healthy relationship because of the underlying family dynamics.
Thank you everyone for your answers!
Can someone please explain why incest can't be a healthy relationship like any other if both parts are okay with it and love each other? (I don't know if it's the case for this manga but I wanted to take the opportunity to ask)