This is a pretty complex subject, mostly because the laws governing it were written shortly after WW2 and haven't really been updated except to say "this applies to software too, not just books and motion pictures".
Legally, in the US at least, they absolutely can sue you for it. Unless your work is actually a parody* or a scholarly analysis, using any part of their work in yours is illegal. However, being a civil infraction, not criminal**, the owners of the copyright have to choose to sue you.
If you're just posting them online, almost all choose not to care. Worst-case, they'll ask to take it down because it's porn, but most don't even do that. If you're charging money, they may take a dimmer view of it, but still might not sue (and if they do, they'll usually settle for you just taking it down rather than trying to wring money out of you).
But if they do decide to sue? You're not going to win.
The rule of thumb is still "don't charge for it unless you made all the models and sounds 100% from scratch", which puts you (legally) on par with things like Palcomix "parodies". Otherwise, make it free and hope they don't have a stick up their ass about being family-friendly.
But even if it's free, your defense isn't the law, because the law is absolutely on their side. Your defense is the company not wanting to piss off fans by acting like dicks.
* A "porn parody" is rarely a protected parody under copyright law. Just having the characters fucking doesn't qualify as a parody legally, you have to actually be making a statement of some sort, to put it (very) simply.
** Thanks to some other laws, they can probably find some criminal charges to slap you with, but the police don't give a shit about this and won't charge you of their own accord.
Obligatory postscript: I am not a lawyer, this is not official legal advice, this is just my layman's understanding of the law.