It would depend on the laws of the jurisdiction.
In most jurisdictions with that laws derived from Merry Olde, if you are married and you have sexual intercourse with someone to whom you are not married, then you have committed adultery. Usually there is no penalty, but it usually permits the offended party being able to immediately seek a divorce, rather than wait a certain period following separation.
In a few jurisdictions, there is a criminal penalty for adultery: in Canada if you are physical committing adultery in a house where a child lives you open yourself up to a charge of corrupting the morals of a child (but I’ve never come across this charge actually having been made in the context of a separated couple – perhaps it might have been in days gone by); in South Carolina you can be fined or imprisoned for committing adultery (I don’t know if this actually happens – somehow I doubt it); in Nigeria you can be stoned to death (but don’t blame this on English based law – the stoning stuff results from the superimposition of Sharia law in the north of the country that for the most part runs on English based law).
Adultery used to, and depending on jurisdiction still may, affect corollary relief such as custody, access, child support, spousal support and property, however, more and more jurisdictions are moving to no-fault divorce in which adultery has no effect on collateral relief. Put together a parenting plan based on what is best for the kids, rather than based on who had sex with whom. Base child and spousal support on need for support and ability to pay support, rather than on who had sex with whom. Base property (assets and debts) equalization or division on equity (often using a formula to reduce litigation), with little or any regard to who had sex with whom. Court fights dealing with adultery usually end up with he-said she-said squabbles that are financially ruinous without helping figure out what would be best the best resolution (e.g. she says he fucked around on her, he says she cut him off after she bore their child, she says she didn’t cut him off – they have three children, he says the DNA test prove that one of then was not his, and on and on and on, with little or anything helping the judge make a decision on the corollary matters in which who committed adultery isn’t particularly relevant).
Depending on the jurisdiction, if a separation agreement has been made, post-separation adultery will have no effect on the corollary relief, although if there is a material change in living arrangements post-separation agreement, that may have an effect on custody, access, child support or spousal support.
Most health benefits contracts usually will not cover more than one spouse at a time, and usually will not cover a person’s divorced ex-spouse, so many folks delay getting divorced so as to maintain spousal health benefits as long as possible, and only get divorced once they want to put a new spouse on their health benefits (hoping that before they put their new spouse on their health benefits, their non-divorced ex-spouse will get a new spouse and go under that new spouse’s health benefits).
Conversely, depending on the jurisdiction, people may wish to divorce earlier rather than later to avoid problems should they become incapacitated or should they die. It can get really complicated depending on the circumstances, for sometimes divorce law does not fit cleanly with estate law, so I won’t go into I here. It’s something that usually can be taken care of when making a separation agreement.
There is no limitation period on divorce other than death, but usually there are limitation periods on corollary relief triggered by divorce (again, depending on the laws of the jurisdiction), so usually it’s good practice to settle all matters prior to seeking a divorce, and from a practical standpoint, the sooner parties work out a separation agreement, while facts and figures are still fresh, the less buggered up things will become when they are arguing about the fair market values of various assets a few years after the date of valuation.
Since it is normal and emotionally healthy for separated people to move on with their lives, including finding new partners, there is a lot of post-separation adultery, but provided that you don’t live in a third world rat-hole like northern Nigeria, it won’t make much of a difference in your life.
So Napier, you don’t have much to worry about having had sex with someone other than you married spouse following separation, other than having committed the sin of fornication with your new partner and the sin of adultery against your married spouse. Fortunately, this sort of religious clap-trap is gradually being removed from Anglo countries’ laws.
Bottom line: assuming that you live in a typical English law based jurisdiction, yes, you committed post separation adultery, and no, no one other than religious nutters could care.