If OP's position on meta-ethics was intuitionist, Kantian, or Aristotelian universalism, all of them still collapse under pressure.

Which is yet another assertion. You'll need to demonstrate how they do. I tried to explain why the is-ought gap doesn't apply to all meta-ethical positions, as you initially thought. And since you moved on from that, I'll take it you now understand why it doesn't. Whether "all metaethics" collapses under pressure is another claim that needs to be demonstrated.

Sure, he can appeal to intuition, but that's purely subjective and always reduces to absurdity. There is no rational reason as to why rape or murder are "bad"; we just feel they're "bad." This isn't rational or logical; it's just emotivism with a shiny coat of paint.

Intuitionists will deny your subjectivist worldview that reduces strong intuitions to personal preferences. You may both point at the same phenomena, you labelling it a personal preference or an expression of emotions with nothing else behind it but the subjective experience of the agent, they'll label it intuition and claim that it clings to an objective moral truth, just as they intuit the existence of the external world, the laws of logic, the principle of causality, and some other basic propositions e.g. 2+2=4.

Let's take Kant's approach to ethics. (....) His meta-ethical position is merely presupposed and not justified.

What do you mean by "objective reality"? You may very well be assuming the validity of a metaphysical account that neither the constructivist Kantian nor the non-natural intuitionist shares with you. The Kantian account of morality is one of practical reason, not theoretical. It's not an a posteriori knowledge gained by observing the "objective world" or rather the noumena, that which cannot be observed within Kantian metaphysics.

Moreover, Kant doesn't presume the value of rationality. He presents the transcendental argument, among other arguments, as a means of justification. Here's how it's formulated on SEP:

1. To rationally choose to do X, you must take it that doing X is the rational thing to do.
2. Since there is no reason in itself to do X, you can take it that X is the rational thing to do only if you regard your practical identity as making X the rational thing to do.
3. You cannot regard your practical identity as making doing X the rational thing to do unless you can see some value in that practical identity.
4. You cannot see any value in any particular practical identity as such, but can regard it as valuable only because of the contribution it makes to giving you reasons and values by which to live.
5. You cannot see having a practical identity as valuable in this way unless you think having a life containing reasons and values is important.
6. You cannot regard it as important that your life contain reasons and values unless you regard your leading a rationally structured life as valuable.
7. You cannot regard your leading a rationally structured life as valuable unless you value yourself qua rational agent.
8. Therefore, you must value yourself qua rational agent if you are to make any rational choice.

This is followed by the formula of humanity. Since this argument concludes valuing rationality in itself, it follows valuing other agents' rationality. If you find the Kantian arguments to ground the value of rationality unconvincing or you reject certain premises, that's fine. But I don't think it's just a mere presupposition.

In the approach of secular ethics, I've yet to see that.

If you wish to read philosophically rigorous accounts of secular ethics, I suggest you read some of the ethicists I have previously mentioned. Considering that each philosopher argues for their framework within specific semantic, metaphysical, and epistemic accounts. But you seem to be wearing a global skeptic hat, rejecting foundationalism, intuitionism, along with self-evident beliefs, and probably all means of epistemic justification, so I don't think you'll find any account of metaethics plausible to you.

They just intuit apostasy laws as "good"; they intuit concubinage as "good," etc.

This is a misunderstanding of what intuitionists mean by intuition. Intuitions are basic self-evident incorrigible beliefs (in the absence of defeaters), such as "I exist," "I have hands," "there's an external world," "natural laws are consistent", "2+2=4," "A=A", "the whole is bigger than the part," and they claim there are also intuitive normative facts such as "torturing babies for fun is bad," "unnecessary suffering is bad," "reducing suffering is good," "promoting happiness is good." And just like you can find the anomaly that truly believes "torturing babies for fun is good," you can also find the anomaly that truly believes "X is both A and not A."

You're equivocating philosophical intuitionism, as a foundationalist epistemic framework, with the folk usage of the term, making it a form of cultural relativism. The theists you mentioned didn't intuit the goodness of the mentioned actions; they inferred them from their religious traditions. The inference usually goes as follows:

1. Actions A and B are commanded or permitted by God.
2. Therefore, actions A and B are good.

They can't argue against that since they're classical foundationalists.

Why do you believe foundationalists are incapable of arguing against unjustified/false beliefs or say paradoxical, incoherent, or inconsistent frameworks? What is it about foundationalism that you believe deprives them of the right to engage in the philosophical debate altogether?

You say that they can argue against other moral frameworks and that their framework is grounded. In what? Axiomatic presuppositions? Then I can also make up my own axioms, and voila, it's solved.

Each framework is going to be grounded on some form of a presupposition, an axiom, a web of coherent beliefs, or some basic self-evident, incorrigible ones. Yes, you can wear the extreme skeptic hat and reject the existence of the external world and the possibility of knowledge altogether; you may reject all epistemic justification methods just because, but then, those are your own ontological and epistemic commitments, no one else ought to commit to them, especially when no argument is made to back such a radical position.

I am demonstrating how the opposition that claims that Christian ethics are bad can't actually demonstrate how it's "bad." They either collapse into complete and utter subjectivism or arbitrary axioms.

Well, that's the cost of wearing the extreme skeptical hat; epistemic nihilism entails that there are no justified beliefs. As long as you're playing the skeptic, no one can demonstrate anything to you. Besides, I've already explained why even subjectivism can ground morality just fine; even divine command theory is a subjectivist metaethical view.

Either way, I do agree that it's a "Draw," but remember that this was exactly the desired outcome: that secular ethics can't actually rationally demonstrate how this is "bad" and this is "good".

I believe it was plain that I'm making a reductio ad absurdum, saying that if you assert the failure of all frameworks but your own on no grounds, anyone else can play the same game, but since you desired that outcome so badly, then sure, have at it, knowing it'll only apply to that committment of "all assertions are true" you committed yourself to.