A common provision in many separation agreements is one that allows the parties, after the agreement is signed, to live as though they were "single and unmarried." These provisions are always a little confusing because, after all, the parties are NOT single and unmarried–at least, not yet. So, what do these provisions mean?
These provisions are probably more psychologically beneficial than practically beneficial. By the time the parties have reached the point that they're signing an agreement, the marriage is ended and husband and wife are equally ready to move on. Though no divorce decree has yet been entered, there's very little hope for the marriage. In most cases, it takes months before an agreement is reached, and in some cases, it even takes years. By that point, it's time to move on–and being faithful to the marriage is pretty irrelevant.
Since you have an agreement, you probably aren't risking very much by choosing to act like a single woman. You're still married, so technically sleeping with someone else is still adultery, but it's not as risky as it would be without having an agreement in place. Of course, you should also keep in mind that adultery is a misdemeanor in Virginia, but it's rarely (if ever) actually prosecuted. However, if you're entitled to receive spousal support, the agreement has already set the amount and duration–so the agreement protects you by preventing your choices from preventing you from receiving spousal support.
You're also protected from the disapproval of the judge, who could (theoretically, at least) award your husband a disproportionate share of the assets as a result of your adultery. Adultery is a pretty serious negative nonmonetary contribution to the marriage, and our equitable distribution statute allows a judge to consider this when determining an award of property. Once you've entered into an agreement, you've created a binding legal contract awarding you a specific share of the marital assets–so no judge will have to do it. Though you'll likely still go in front of a judge for your uncontested divorce hearing, by that point the judge is just verifying that you meet the requirements to be granted a divorce. He's not looking to divide the marital property, since you've already done it yourselves.
As an attorney, I have to tell you that no agreement can change the fact that any sex outside of marriage is adultery. The agreement also can't change that what you've done is, by Virginia's standards, a crime. However, these provisions are very common, and do protect you against the two biggest risks that you would face if you committed adultery without an agreement: (1) the loss of spousal support, and (2) the judge's censure.