While it's true that many men experience higher-than-normal arousal when their female partner has sex with another man, the cuckold thing is only one permutation - and not a particularly healthy on in the long term.

Both partners will experience increased interest in the other when the other has external sexual adventures. Although obviously women will have hundreds of times more opportunity (and far greater ease) with regard to finding men, it is nevertheless important for the long-term health of the couple that there should be some balance. The man should, albeit inevitably infrequently, have some opportunities for adventure also. Otherwise significant emotional downside may ultimately eventuate. What is exciting today can become resented tomorrow if it is always the same thing without any hope of alteration.