Don’t feel pressured by the notion of “let’s be together” or “get over the past” because people want to take their time to reflect and mark the end of a relationship when it is in danger or breaks up. If you really want to get to know them, go ahead and do it. If they complain about their marriage while you are with them because of the past, give them time because they have not yet healed from their memories and past injuries.
You should realize that they are not yet prepared for a new relationship, especially if they speak negatively about their previous one. It’s possible that they started dating you in order to prove to the other person that they are capable of having a second lover if you notice that they struggle to decide whether or not to stay with you when they fall in love with someone unrelated to their spouse.
Therefore, take care not to jump into a relationship just because they say yes as this could make your time in love difficult.