If you're adults, you're not bothering anyon and you're not harming yourself, then do whatever you want imho. I think it might be taboo, but not fucking illegal irl
Personally, I don't get too exited about mother incest, probably because I didn't get an incest fetish till I was in my 30's. Old enough to not feel the younger perspective so strongly. It's still hot like any incest, just doesn't rev my engine as much as brother-sister or father-daughter does. Mother-daughter has the added bonus of lesbians, so I do love that one.
There's a lot of discussion above on why the line is different for fantasy and real life. Fantasy is fantasy, and I don't see anything wrong with it so long as it stays fantasy. I think real-life incest should be considered wrong not because it's inherently evil, but because the potential for abuse is too damn high. If people could all be actually trusted to not take advantage of others, I feel like such taboos would no longer be needed. Too bad it's impossible for the general population to be that trustworthy.
I've seen some people online claiming they were IRL the younger family member in an incestuous relationship and looking back when much older, they don't at all feel taken advantage of. In those cases, I'm really not morally bothered by it (assuming it's even real). In cases where they later do feel they were taken advantage of, I do feel morally disgusted. It's not something people talk openly about so we can't accurately know how common either is and you can't know how people will feel when older, so it's definitely best to be safe and just keep the whole mess taboo.
Option 3: If we were strictly talking about porn. It being taboo is 99% of the reason why it's so hot. I hate stories that neuter the fetish by normalizing it. Like all those stupid doujinshi plots from Japan where incest is sanctioned, encouraged or enforced by the government cuz "mah declining birthrates".
IRL: I'm all for people fucking their moms but I would never, EVER want to fuck my own.
I would fuck a mother daughter combo, but that's probably the most actual incest i would ever do. And even then, it's so weird to think of doing it irl idk if I'd be able to lol