Mammals are driven to sexual excitement by Pheromones released when the female goes into estrus. It is natures way of assuring the old cow gets bred as well as the young heifer.
Humans are driven to sexual excitement by sight of a good looking naked lady. Otherwise Playboy would have come with a “Scratch and sniff” card instead of a centerfold.
That is why there are no homosexual animals. Other male animals do not release a Pheromone to excite the males.
I worked for a blind guy for 25 years. He has 2 kids. I wonder how he managed to get sexually excited. By the way he has a beautiful wife.