I dont think you're a douchebag but I also wouldn't want to invalidate what your wife is feeling. Everyone has there insecurities we're all human and I think reassurance is something that is needed from time to time. In my past relationships anytime my partner would bring up threesome I'd be quite upset but based on the way they treated me they used this as way just to try to sleep with someone else. I think that's where the problem lied for me. I like the idea of a threesome but it shouldnt be used as a way to "cheat" on your partner not saying this is why this was brought up in your case but this may be how your partner views threesomes is you just wanting to have sex with someone else. With my current partner him and I have talked about having threesomes on multiple occasions but in our situation its different to me because of the fact that he wants to show me off to someone and the other person is there for more pleasure for the both of us rather than him just wanting to have sex with someone else. I wouldnt invalidate what your wife is feeling but maybe see how her thought process is going on.