I find myself not interested in sex with my wife but feel that way about my neighbour's wife

The first thing I would like to suggest is that this is about more than sex. In other words, it sounds to me like there is a promise in the possibility of a sexual relationship with your neighbor's wife that is absent to you in your own marriage. Please note that this is not about blame: blaming you, your wife or anybody else would actually serve no purpose so I'd like to avoid it. Back to your question: what is this desire pegged on? What desire would sex with your neighbor's wife fulfil? What would sex with your neighbor's give you that feels absent in your own home? As I mentioned, this is about more than just sex. If this was just about sex, you would be (thinking about) sleeping with many other women.