Most all straight men are aroused by the sight of big dick. That's why straight porn is full of big dicks going everywhere.
I never sucked a dick, because I am a bull (I'm hung bigger than most porn studs).
That said, I love eating pussy and watching lesbian porn, but another sexy bull turns me on secretly. I just don't admit it to others.
For me, the fantasy is not so much about sucking a bull's dick, but just feeling up his hard body everywhere. Just soak up his masculinity and prepare the woman for him to enter her. Taking his clothes off, and making him feel good (massage). I'd be more into jacking his dick off before he pumps the woman's pussy. Not sure why sucking dick just doesn't arouse me.
We're all wired differently.