When God says "don't do that" about *anything* the first and best answer is to say "Aye aye, Sir" and obey. He said don't do it and that's all that's necessary, our job is to shut up and soldier. However, Romans 4:15 and 5:13 are particularly instructive to your question, in that where there is no law there is no violation and no sin imputed. Prior to the law, Abraham marrying his sister was not a problem. With the coming of the law, from that point on it has been forbidden and that still counts for New Testament times. Observe the violation of Leviticus 18:8 that was mentioned in 1st Corinthians 5 (and his Dad was still alive- it was incestuous adultery within the church). That said, it isn't a bad thing to examine the prohibitions for what they can tell us.
In this case I think the better question than why it's forbidden is why the mother/daughter thing is punished by burning them *all* with fire but there is no prescribed punishment for the man who takes a woman and her granddaughter or for the man who marries sisters. The dichotomy between Leviticus 18:17 and Leviticus 20:14 perhaps allows us to see a clearer picture than might be presumed by just looking at Leviticus 18:17.
If we look at the context, within the portion of the statutes prohibiting incest we find two that apply specifically to polygyny. Observing the "uncover nakedness" wording, we see the prohibition on marriage to sisters and the taking of a woman and daughter/granddaughter are characterized as incest. The question at that point is who is uncovering the nakedness of whom? That is, who is committing incest? Given that the husband is not biologically related to them (that would already be forbidden as an incestuous marriage), the incest issues can only apply between the women, but the man is part of it.
(Verses 17 and 18 are particularly uncomfortable for many because they presume sexual contact between wives in a poly marriage. Which raises the point that sexual contact between women is not forbidden and the prohibition on marriages that would involve a case of sexual contact between blood relatives (the women) emphasizes that.)
When looking at the incest statutes in total, there appears to be the glaring omission of a father-daughter prohibition. Comparing Leviticus 18:17 to 20:14 we see the possibility of the case in which contrary to the general command in verse 6, the man marries his own daughter in addition to her mother. And now we can see a father-daughter prohibition are as well as where the burn them with fire comes from, because they hit the incest trifecta.
The wording of "uncover her nakedness" can apply to more than sexual acts, although the context is sexual. By way of contrast, it is not forbidden for a mother to see her daughter or granddaughter naked, to bathe with them or sleep in the same bed with them. Neither is any of that forbidden in general for sisters. Move that to a sexual context involving a man and the prohibition applies even though it might not involve physical contact. That means everything hinges on the word "take" which is easily interpreted as forming a sexual relationship. As to whether it extends to marriage, that's arguable.
However, in looking at Leviticus 20:14, where we see the word "marry" used, we are most definitely talking about sex within the context of an incestuous marriage. That triggers a condemnation of immorality with the punishment that all of them being burned with fire. Is the difference that the burning with fire is a case in which the father takes his own daughter? I suspect it is. However, it's certainly possible that it's worded that way in order to include those cases in which the man marries his stepdaughter in addition to her mother. While Deuteronomy 27:23 specifically says "Cursed is he who lies with his mother-in-law" I suspect that far more of the time it would be a case of the daughter rather than the mother. With respect to the man marrying his daughter or his step-daughter, I see a betrayal of authority, responsibility and trust in addition to the incest, but that's just my opinion. The Word says what it says.
In the end, I think the implications of Leviticus 18:17-18 are important in terms of understanding Romans 1:26-27. The reason is that Romans 1:26 cannot be a negative reference to women who have sex with each other, but rather to women who reject marriage and motherhood in violation of God's design and the command to be fruitful and multiply. That means the men who reject marriage and fatherhood are likewise rejecting God's design and in addition to that, while rejecting women they violated the law prohibiting men lying with men as with a woman. In other words, those passages are not about homosexuality, they are descriptive of a "degrading passion" that causes both the men and women to reject God's design for marriage and family. That opens up entirely new vistas in not-PC discussion because it puts feminism front and center as the degrading passion.
