Well OF COURSE they are!
Well maybe some folks mostly enjoy the "being babied" aspects of this (the ABs) but most of the rest of us (the DLs) really do love our diapers and it's in a sexual way. That's the definition of "fetish"--it's being sexually attracted to some inanimate object, whether it's diapers or panties or sissy clothes or whatever it is.
And my wife--even though she knows I love my diapers and I love her panties--really doesn't understand this at all. I love my diapers whether she's participating with me or not, and I love her panties whether she's wearing them or not. It's the object itself and being near it that provides the turn-on.
And in my case, with cloth diapers, it works on every level. I love the way they look, stacked up, hanging up to dry, on me, in the diaper pail. I love the way they feel, all fluffy and soft when I put them on and all warm / wet / loaded after I've been in them a while. I love the way they smell. I love the way they sound when the plastic pants are crinkling! They appeal to all of the senses in such a wonderful way.
And I love the "progression" of wearing them. In a way very similar to going out on a date and later having sex (you get in the car, you pick up the girl, you go out and have fun, you later start cuddling, it keeps building until you finally go hop in bed and make love), wearing your diapers starts off with the decision to wear them. Then you get them out and see them / feel them / smell them as you put on the powder / baby lotion and put them on. Then you wear them and wet them for hours as you get them soaked (and if you're into it, loaded). All along your anticipation is building to what you know is coming (no pun intended). Finally when you're ready, you lay down and hump them until you get yourself off (or however you do it--each of us have our own unique ways we do things). In my case, soaking wet, thick, flannel cloth diapers with a big shot of Baby Magic lotion in them feel every bit as good to me as being inside of a girl and the orgasms are every bit as intense.
So yes, at least in my case, it's a sexual thing--though I also do enjoy the other aspects of it (being treated like a helpless toddler, the humiliation, etc.)
And as pointed out below, yes, for me just hearing the word "DIAPERS" is often enough to get me excited. Often when I'm getting myself off in mine I will just start saying out loud "Diapers! Diapers!!!! DIAPERS!!!! DIAPERS!!!! I'M WEARING DIAPERS!!!" and that just adds to the enjoyment!