Oh that's simple. Why dick is fascinating.....
Let me explain this to you from a gay guy's POV and why I like dick.
They Grow: Dicks are fascinating because they grow when excited. If a man is into you or the experience, his dick will show it. It feels nice to have a warm nice dick fill with blood and grow in your hand, inside your mouth or even just feeling it "fill" your ass or pussy.
They Are Fun To Play With: Dicks are sooo fun to play with. You can slap them against your hand, face, ass, chest, rub them against pillows or place them inside "safe objects" to enhance the sensation. You can even sword fight with other men. For that alone, dicks are just plain fun toys. I'm gonna play devil's advocate here because even "straight" men like the idea of a dick with as much "ass play" they try to cross over into straight porn with pegging. While they may not be attracted to men in general they sure are attracted to what men have. OOF!! They like the feeling of their ass being filled. Even if it's fake it was still modeled after a real dick.
Variety: Dicks come in a variety of shapes, sizes and colors. Walking into a room full of dicks is like walking into a candy store with all the choices of candy from around the world. Dicks all have the same function but they have a mind of their own per individual. Some curve to the left, some curve to the right, some stick straight out, other aim towards the sky. Some are small and gentle, others are heavy and will rearrange your insides if you aren't careful. Some have a shield, others take things "head on". Black dicks, Euro dicks, Asian dicks, Aussie dicks, Polynesian dicks...Just lay them all out please. Dicks are just dam beautiful.
Spasm Machines: Ever jerkoff and feel your dick spasm? Imagine feeling another man do that in your mouth or ass. It is truly amazing to feel another dick spasm. Nothing can compare to a real dick in your mouth or ass...nothing... While machines and dildo do offer that edge.. Having a living breathing man pumping and fucking you where you can feel his warmth, sweat and other body parts (grabbing his ass) is all too rewarding.
Bonding: Dicks bring men together. You don't have to be gay or bi to jerkoff with another man. You're just beating your dicks to something you like...if you are watching straight porn and like titties and pussy, you can jerk off with your friend in the room, both cum and that's it, without even looking or touching each other. Dicks bring men together.
Cum: Yes, Dammit. Dicks shoot out cum. Sperm is fascinating. Seeing a dick erupt is hot. The idea of sucking, fucking or jerking another man's dick works based on giving someone pleasure through your skill, and working for your reward. Ever dick is different and some guys can cum easily while other can't. Putting in work on a dick and watching it cum is a truly rewarding experience. I used to be terrified of sperm. The first time I saw it come out from another guy I jumped back. That changed over time of course, even to the point where I randomly got on my knees after getting fucked and told the guy to give it to me. He came all over my face and I laughed. It was the first and only facial I had but it was interesting to see his reactions during it.
A man can have an orgasm without actually cumming which is why seeing a dick cum has turned into this sort of thing that closes the deal. It's satisfying to see the results after a job well done.