For me, I despise the dirty old man trope for a few reasons.
1) It requires more suspension of disbelief. There seems to be this pervasive trope in avn land that "if a girl gets slutty enough, she'll fuck anything." I realize this is in the realm of fantasy, but that's so far removed from reality it kills the vibe for me. That's simply not how women (or people) work. To anyone who doubts or has trouble following me, picture that logic applied to men or yourself as an example. If someone told you, "I want you to become man-whorish enough that you'll want to fuck this toothless saggy old woman", you'd be like uhhhh, gross, not in a million years would I want to fuck that bag of bones. For me things that are semi-believably are far hotter.
2) While trying to remain cordial, and not accusing anyone directly; I've a suspicion many of those (but not all) who like the "gross old bastard" pairings aren't honest with their reasoning, and a large part stems from an undercurrent of resentment towards women, and enjoyment from seeing them "defiled", abused, or otherwise taken advantage of. This is the polar opposite from what makes a hot scene for me. I tend to come at these things from the swinging/sharing angle, and I'd much rather a wife or gf have a partner who she thoroughly enjoys. That's not to say only "safe" options, by all means, if she enjoys it, let her have raunchy aggressive sex with the guy. But with the [gross old bastards], there always seems to be an undercurrent or subtext of control/abuse/"putting one over" on the girl.
3) I do understand what people are saying about the more illicit and taboo aspect often being more appealing. However, for me, in the swinging context, that gets inverted.
By that I mean yes, comparing a single girl banging a hot guy vs a single girl banging an ugly old hobo, the former is more predictable and the latter more shocking, taboo, and thus perhaps erotic.
However, when we're talking about a girl in a loving relationship, while the ugly old hobo can be written off as more of a "crazy party stunt", or a novelty, an attractive guy banging her brains out seems much more taboo and illicit, especially if she remains 100% committed to her partner. That creates a very strong erotic juxtaposition, like "she's in love with her bf yet screaming like a banshee with this other guy". <---do note that contradiction-induced taboo goes out the window if it's in the context of cheating, which is why I love swinging/sharing and hate ntr; a girl cheating on her bf with a desirable guy isn't shocking, it's predictable.