My point is that the parents are a man and a woman and this is talking about potentially two men and two women. We know that if these things happen randomly (two genes inherited from one parent, even if from the other chromosome), that it causes severe developmental problems.

The genes in sperm are physically different than those in eggs. It's called methylation and I gave you an example of a disease caused by it going wrong.

I never said anything about the likely safety of the technology, so I'm not sure what that has to do with it?

The genetic mix from parents is randomized. Always. Both good and bad results are equally the result of a random process initiated by the parents. A combination of recessive genes to cause disease is not more or less random than a combination of healthy genes. The parents roll the dice, the child gets the result.

In other words: The ethics can't proceed from first principles around consent to experimentation. It has to include, as you say, empirical questions about the technology in question. But that's a different sub-thread.

If I had to say anything about that: I think this is one of those things that is just going to happen whether or not anyone likes it. The fact that it has already succeeded with mice + the massive applications... it's just going to get tried, it's just a matter of which country it happens in. The fact that it could potentially be used by same sex couples is a total side note to how strongly this will be pulled along by the same existing demands that drive IVF currently, the potential is just too powerful. So I imagine we're just going to see eventually.

I wonder if the next step for this will be to develop techniques to modify the methylation of genes to make them compatible.

At some point you're going to want this if you're trying to pick specific genes from people or if you're doing things like making people with dozens of parents.