Why is sex fun? If you've ever had an orgasm (even if only by masturbating), especially if you've ever had a mind-blowing orgasm, then you'll know. There are no real words to describe it, and everyone's experience is different, but it feels GOOD!!!
Why is sex enjoyable? Because it's a celebration of love. It's about wanting to be THAT close to another person. It's not about someone just "wanting attention." Just wait until you meet the right person, then you'll understand.
Why is sex so disgusting? Well, when you focus on the literal, physical aspects of sex, it does sound pretty disgusting, doesn't it? Two naked bodies rolling around under the sheets, penis inside vagina, licking each other's genitals where the pee and poo come out, eating/drinking each other's bodily fluids, etc. I don't know how old you are, but the whole thing typically grosses kids out. But, at some point in our teens or young adult years, it stops being gross somehow. Somehow or other, we just reach a point of understanding where we just get why this activity is pleasurable.