Maryland is a no-fault state, but that just means that it has both fault and no-fault grounds for divorce. “Circumstances surrounding the estrangement of the parties” (why you split up) is one of the factors the court can consider in deciding alimony and property division.
If you start dating after separation, then it cannot be a factor in why you split up. However, your spouse may try to prove that the dating started before separation. Whether it did or did not, that means a lot of time, trouble and expense at trial. And the judge has no truth detector at the bench. I have heard judges say the parties spend 90% of their time at trial on fault and the judge gives it 10% consideration.
That’s why I tell my clients not to date. Of course, many of them do not listen to me. So I tell them to be discrete and remember that the guy next to your table at the restaurant may be a private eye with a video recorder hired by your spouse.
A Separation Agreement changes everything. Adultery is still adultery because you are married until you are divorced. But now it is adultery without legal consequence, because property and alimony are already determined. Adultery is immediate grounds for divorce, so it might speed up your divorce, but it will not change the agreement. So you are free to date, and live as though you were single, once the agreement is signed.