If they were part of the same family
As long as it was safe, I see no harm
I don't really agree with having sex with relatives but if they're both consenting adults I see no need to punish either of them.
I wouldn't. It's none of my business what two consenting adults do in their own bedroom.
As long as it's consensual then no, there are far worse crimes out there innit
As long as they're on contraception and it's protected sex, I wouldn't
We're not responsible for protecting the minds of adults. If a 20 year old woman really, really wants to **** her grandad (and if he's okay with that), I'm not going to be the one to send a policeman in to drag them apart. Same goes for brother and sister, cousins etc. We don't have to understand each others desires, just respect the rights of adults to make their own choices.
Only if children are going to be produced can the state claim any legitimate interest in interfering. And nowadays that's absolutely not synonymous with sex (and often not even with marriage).
In general, not the state's business. The question of children is a very interesting one though. If you think on account of potential genetic problems (which by the way are still very unlikely with cousins or further, though several generations of inbreeding will increase the likelihood IIRC) we shouldn't allow children to be conceived in these circumstances, would you also ban people with inheritable genetic disorders from reproducing? Would you force the abortion of fetuses that will end up as severely disabled babies? I would be inclined to say no in these cases and thus I can't reasonably see how the incest case is different in this regard.
One thing to note though is that there are obvious questions of power imbalances that need to be brought up. I'm extremely socially liberal, but I have to take issue with the constant throwing around of the term "consenting adults", as if the age of 18 bestows magical powers of self-responsibility as soon as one attains it, as if it has a fundamental moral basis rather than a culturally dependent one. Imagine an 18 year old still lives with their parents and is financially dependent on them. Can you say that this 18 year old having sex with a parent is morally justifiable because it's simply a case of "consenting adults"? I doubt it.
So in general I would advocate no prison time or fine, but I recognise that this is not so black and white as "consenting adults, state shouldn't care".
If it results in a pregnancy, I would probably give them a fine due to the chance of producing children with birth defects.
what? why would locking them in a cage do any good? it's not my business and I don't give a **** - consent is the only thing I care about