If you'd humor me, I'd like to get a few answers to this question from anyone that wants to answer it and I'll reply with my thoughts.
Why is stealing immoral?
Why is cheating on a person you are in a committed relationship with immoral?
Why is rape immoral?
Why is murder* immoral?
(if all of these things are immoral for the same reason, you can lump them together)
What about suicide? Is it wrong, sometimes wrong or never immoral?
If an act is immoral, is it wrong? are immoral acts bad?
*Murder is the intentional and unjustified killing of another human being without their consent, conducted with malice aforethought.
I base my measurement of immorality on degree of harm caused to another person. This is not an exhaustive explanation for what is immoral, but it covers the vast majority of it.
Which, I suppose, is a way of ducking the question, because then you might ask what is harm?
But that way lies madness and chaos, so...
All of those things are immoral because they cause some degree of harm to another person.
Case 1: Stealing is immoral because you are harming another person by taking things they own and/or need. Arguably, people with more stuff and the resources to acquire replacement stuff are harmed less by theft. But still, they are harmed.
Case 2: Cheating on a person breaks their trust. Whatever your relationship agreement is, I am assuming cheating breaks it.
Assuming you are in a committed monogamous relationship with them, it harms them because you both agreed to that monogamous relationship, and you are betraying their trust, not to mention harming them emotionally because they are operating under the belief that various emotions they feel are shared only with you, and you reciprocate them. Definitely mental and emotional harm, possibly more.
Case 3: Rape is immoral because you are significantly (and often permanently) harming another person physically, mentally, and even beyond that - to one degree or another, in almost all societies, being a victim of rape potentially harms a person forever, even if they personally recover entirely, because it changes how some or all of that society views them.
In some ways, I am tempted to think rape is arguably worse than murder.
Case 4: Murder is immoral because you are harming a person to (arguably) the ultimate degree, by ending their life, preventing any future actions they could have taken, for good or ill. But we will never know, because they are dead.
Case 5: Suicide is wrong because it harms the people who love and/or rely on you. There are cases where it may be less harm than remaining alive, but that's a case-by-case thing.
Immoral acts are bad because they harm another person.
If you, 5 minutes from now, destroyed the universe, that would be immoral because you significantly and permanently harmed a number of persons we cannot grasp currently. But if you have that power, perhaps you can, and would know how much harm was caused.
Immoral acts are not simply good or bad, however. The degree of how immoral and/or bad an action is, depends on how much harm is caused, and how much recovery can occur, and/or recompense be granted.