In Virginia, you're married until you're divorced. If you're separated, you're not divorced. If you're married and you have sex with someone who is not your spouse, you have committed adultery. In Virginia, adultery is a Class 4 misdemeanor.
Even if you and your husband are separated and you (or he) has sex with someone else, you've still committed adultery because you're not divorced. That being said, there IS a difference between pre-separation and post-separation adultery. Why? Well, pre-separation adultery is generally believed to have been the reason for the breakdown of the marriage, whereas post-separation adultery happens after the breakdown of the marriage. What's the difference? In many cases, to a judge, the difference is significant, because post-separation adultery doesn't carry the same weight in terms of fault.
Why does this matter to you? The way property is divided in Virginia (we call it equitable distribution) allows a judge to take into account the contributions of the parties, both monetary and nonmonetary, to the marriage. In the event that one party has done something with such serious nonmonetary repercussions, theoretically at least, the judge can use those actions to justify one party receiving a greater award of the marital assets. Notice that I said "theoretically." Normally, fault doesn't play too great a role in the distribution of marital assets, but under equitable distribution it could.
Additionally, regardless of whether you committed adultery before or after you and your husband separated, adultery will prevent you from asking for spousal support. So, if you're hoping for spousal support, you should probably just wait until you're divorced before you have sex again.
For most practical purposes, the difference between pre and post separation adultery is only slight. You're still married, but it's still adultery, it's still a crime, and it could still bar you from receiving spousal support.