What an awesome villain and character! I seriously enjoyed the film and was just wowed by everything about Ultron.

I can certainly see the fan-art, fan-fiction, the fantasies people will have with the character, It's bound to happen.

Is it because he is a pretty big guy? Quite larger than a regular human I will say.

What about that awesome display of intimidation when he said "let's do business"?

He knows he is big and powerful and that makes him dangerous, but also enticing I find.

And a lot of people think that robots, androids, and masked beings are pretty sexy. Marvel does go out of its way to sexualize everything (look at the cast) so why not the design of their CGI characters too? Even Ironman's suit is pretty sexualized in all the films it is present in. (now that could be because Tony Stark is all about that life) but the suit itself could possibly be considered eye candy. Ultron certainly is, and that voice...yes. Good casting on that. Well done.

He also has really big shoulders and a really nice chest. There is no super defining "muscle" like abs or pecs or anything but he was really buff and even I the movie Tony Stark was making comments about Ultron's appearance.

I look forward to see the fan art that is made of the dude, I am very sure a lot of it will be adult (Rule 34) but I have faith that there will be some tasteful clean art as well.

And for the record, speedos/underwear/intimates are still considered general since there is no actual nudity, and an excessive bulge can technically be classified as mature.