My husband enjoys watching me with others (male and female) because he enjoys seeing me pleasured. He has only seen me actually make love to one other person (my ex-gf) and he said he enjoyed that less than seeing me just screw someone. Watching me make love with someone he wasn't involved with romantically felt like intruding to him (even though the two of us were in our own little world so it made no difference to us that our hubbies were watching). If it is someone I'm just screwing, though, he likes to see my enjoyment. It's a different perspective that he doesn't get to enjoy often so it is exciting. He has also said there is an element of pride for him in it - the whole "She's that awesome and she's MINE" thing, which has always been kind of weird to me since he in no way controls my sexual behavior or has had any problems with me doing whatever I want in pretty much any situation... But yeah.
Voyeurism is really common, so no.. I would not say you're nuts.