A naked dude by himself does nothing for me, but I do like my straight porn to include bigger guys.
I think I relate it to my own experience, where my generation was raised with this stereotype that women are the more rational sex and only guys want to fuck, but women just want a man who will kiss their hand and write them love sonnets. When I gained some experience I realised that, while I really love sex, I'll never experience it as powerfully as most women do. Something about seeing a smart, proper, and kind woman just lose her mind while getting pounded hard + deep is a massive turn-on for me.
A lot of my early sexual experience was with girls who were pretty much just using me, and while it's not healthy being objectified for sex really gets me going. Big-dick straight-porn is great since while it's aimed at men, it really objectifies men too.
In reality, I know it's certainly not true that a big dick always equals great sex, but in porn it's a great metaphor for a woman who has really embraced her sexuality.