Of course, it's certainly possible that the answer to this question when posed might legitimately be "Yes!" and then come to not be true later. And it's also certainly possible that the answer "Yes!" may not be meant as truthfully, but you have no reason to suspect otherwise. However, it's also possible for two people to have a difference of opinion about the possibility of opening up the relationship in some way down the line, which is why discussing such possibilities honestly -- at least on the end you can control, i.e. your own -- is crucial.
However, another way to suss out the faithfulness of your future spouse? Looking at their past history. It might have behoved, say, Sandra Bullock to consider Jesse James' reputation as a serial cheater before she married him; I'm not at all blaming Sandra for what eventually broke up her marriage -- James' wandering penis -- but I am suggesting that the rest of us learn from what happened to her. And that is: once a cheater, not always a cheater, but has always been a cheater, will always be a cheater.
