I just think it's innate to be aroused by cocks. If not innate, very very common. Porn for men has hard cocks and cum in it, often several, often being the focus, and the cumshot is the climax where many viewers choose to orgasm. Many guys will say "I'm not aroused by the cock I'm aroused by imagining it's my cock fucking her" but I think that's a post-hoc interpretation of what is happing, which is getting aroused by looking at a cock.