[quote=@SleepingSilence]
So how far is too far for you? Incest? People marrying their 3DS? (if only this was a joke.) How about sex? I feel like when you start going..."Yeah, polygamy is probably sustainable and mentally healthy." I don't even understand why'd you deny anyone love at that point. Go full progressive. For lack of better term.
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I start from the premise that all relationships take place between consenting adults, a DS, however mature and cute, cannot consent. It doesn't really matter if I think its mentally healthy or not. I've certainly met poly people who are in seemingly healthy happy relationships and traditional marriages which are clearly mental deathtraps which shouldn't exist in a just universe. I don't think we can use whether they are 'mentally healthy' as a metric.

At the risk of being somewhat controversial - Incest is actually sort of an interesting case and I'd like people to keep an open mind about it. On the one hand there is an ick factor which society assigns to it but I'm not sure that is all that valid as an objection. On the other hand there are the very real genetic dangers faced by offspring of those unions. Its tempting to disqualify them on that basis alone but there are plenty of genetic conditions which offer similar or worse threats to potential offspring. While we counsel people with these genetic markers we don't have any legal mechanism to preclude them from having children. Then again not all relationships have producing children as a goal. It creates a sort of moral double standard. I'll admit that I suffer from the same social ick factor as most people but I'm not sure that should be controlling.