Sex with spouses
Yes, it's okay to have sex with your spouse after you've separated. It happens all the time. There are three things to be aware of.
First, from a legal point of view, s. 8(3)(b)(ii) of the Divorce Act says that married spouses can live together "with reconciliation as its primary purpose" for up to a total of ninety days following separation without stopping the clock on the one-year period of separation that has to pass to get a divorce. (Once more than ninety days have passed, the one-year period starts to run from the last separation.) Honestly, though, I don't see this as much of a problem. Spending the night with your spouse isn't going to count toward the ninety days unless you spent the night for the "primary purpose" of reconciliation rather than sex, which I rather doubt.
Second, if the legal ground for your divorce is based on your spouse's adultery or your spouse's cruelty toward you, you need to know that you may have been considered to have forgiven or "condoned" your spouse's misconduct if you have sex with your spouse after separation. Under s. 11(2) of the Divorce Act, an act of adultery or cruelty that has been condoned cannot be used as a ground for divorce.
(There aren't any legal problems with unmarried spouses or partners having sex after separation since a divorce isn't necessary to end unmarried relationships.)
Third, from an emotional point of view, you might want to think about what having sex with your spouse will do to the progress you've been making in getting over that relationship and building a life for yourself that doesn't include him or her. Lots of people are able to handle the messiness of sex with a separated spouse; other people find it to be emotionally difficult.
Sex with other people
As long as you're married, having sex with someone who isn't your spouse counts as adultery. If you're separated at the time however, no one except your in-laws or the Pope is going to care.
I suppose it's true that your spouse could claim adultery as the ground for your divorce, but if you've already separated from your spouse, your marriage would seem to have already come to an end for an entirely different reason than your adultery. Apart from this one issue about the legal ground for your divorce, having sex with someone else isn't going to have an impact on how your divorce is handled. It isn't relevant to whether spousal support is payable or not, how much child support will be paid, how property will be divided or what the parenting arrangements are going to look like.
(There's no such thing as adultery for unmarried couples, since you have to be married in order for sex with someone other than your partner to count as adultery.)
As I often tell my clients, there's nothing a separated married person can't do that a single person can, except to get married. Apart from that, a separated married person can see other people, date other people, have children with other people and live with other people.
What's interesting about all of this is that there's nothing stopping a person who's married to someone qualifying as someone else's common-law spouse. Under the Family Relations Act, "spouse" includes, in addition to people who have been married, people who have lived together in a "marriage-like relationship" for at least two years. In other words, if it's taking awhile to get your divorce and you've moved in with someone else, you could have two legal spouses: the person you're still married to and the person you've been living with. Surprise!
I talk about the legal consequences of having two spouses in the Marriage & Divorce > Separation chapter of my website.
Separation is a broad subject. If there's something you'd like me to discuss, please say so in a comment to this post. Click on the "separation" label below to read other posts about separation.