I'm a metalhead through and through and heavy metal is, by far, my favourite genre of all. There's a whole wealth of diverse subgenres within heavy metal and I continually discover new bands and subgenres that radically change my perception of this beautiful genre. My favourite subgenres of heavy metal range from: black metal, death metal, doom metal, drone metal, avant-garde metal, progressive metal, thrash metal, folk metal (some of it), traditional heavy metal, etc. But even in metal there is a fair share of subgenres that I don't like. Power metal usually does nothing for me, although there are exceptions (Alkemyst). Metalcore, deathcore and nu-metal are subgenres that I strongly dislike.
That said, yes, I definitely like other types of music. I'm a huge fan of Gorillaz, I grew up with jazz and blues and I love it, I like some techno/electro music, I enjoy classical music, some indie, alternative rock, rock, punk, ska, industrial, darkwave, dance macabre, gothic rock, folk, ambient, 8-bit music, grunge and I practically worship Lou Reed and The Velvet Underground. I'm also getting into Steely Dan lately.
I don't know which metalheads you've met, but they are not representative of metalheads and their (usually diverse) tastes. A friend of mine listens to some of the most heaviest, filthiest, unforgiving brutal death metal/grindcore, yet he loves the music of Prince, Lou Reed and Gorillaz. Everyone can listen to what he/she wants to listen to, but to limit yourself to one genre is, in my opinion, quite silly.
Sorry for the abominably long post...