I really don't see the problem here. Your relationship with your wife may be good now, but it will probably get worse again. I would just bang her. It is only once a year and is very little effort. Just think about it as helping the poor girl out. Nearly all relationships end eventually, and generally they end horribly. The only relationships that last years and years and years are ones that take a ton of work and I truly doubt that both parties are very happy, or maybe they just lower their expectations for happiness. True love, soul mates, or believing there is only one person for all of us is a fairy tale. Human being aren't meant to be in longterm monogamous relationships. This idea is a social construction. We are much too individualistic to be tied down. I say just do what makes you feel good. Don't let society tell you what to do.