Yeah, I never understood it either. Either have an open relationship with consent, or communicate whatever needs you have that you want to fulfill by cheating properly, and accept that it may not work out if there is truly no way to meet them. I guess I can at least abstractly understand when it happens spur-of-the moment and under the influence of drugs/alcohol, but I still can’t properly put myself in those shoes.
Which I would classify as pretty weird, but not really unethical. Besides, I think the comparison doesn’t fully work - it’s more like, growing a lump on your body somewhere, having it removed, and saying “hey, can I eat that?”. Which I would also classify as weird, but not unethical.