Incest, homosexuality and civil partnerships

Posters on Cranmer's blog have made a point which I think it worth exploring some more.

The initial case under discussion was the case of the two elderly sisters who had always lived together, never married and were now scared that if one of them died, the other would have to lose her share of the house to pay inheritance tax.

On solution suggested was that the sisters should be allowed to form a civil partnership. Under the law which introduced civil partnerships, people of certain degrees of relation with one another are not allowed to become civil partners. The degrees are the same as those used to prohibit marriage of people too closely related. The suggestion put forward was that there was no reason why civil partnerships should be subject to this condition.

The point of prohibiting incest is because of the genetic problems it can cause in offspring. When you have a same sex couple, this problem doesn't exist. Why then do we prohibit incest between consenting, homosexual adults?

Another poster observed that making this argument was homophobic as it undermined the idea that gay people couldn't have 'true' loving relationships.

I don't think it undermines it at all. The Civil Partnership Act recognised that there were many people who were in relationships which were akin to marriage, but who were not afforded the rights and protections marriage awarded to married couples. The law rightly said that not granting these privileges and protections to people simply because they did not fall into 'the norm' when it came to their relationship was wrong.

As homosexuality is not inherently harmful, the state should not seek to discriminate against those who practice it. Homosexual incest is equally harmless, what is the case for the state discriminating in this circumstance?

NOTE: the sisters in the original part of this post are not gay or incestuous. I just thought it brought up some interesting ideas.