We don't know, but it is not a choice, it is a fact of your parent's life. The social pressure to behave in accordance with your birth sex is enormous. Most transsexual people try to do this, in hopes they can do what everyone wants them to do. Often a transsexual person will do something that has a strong gender role, such as join the army or get married, in hopes that they will be forced to do what they do not feel like doing. People love other people.
Sexual intercourse is just one part of that, and it can be a small part.
Just as with the general population, some transsexual people are lesbian or gay. So what looks to the world like a heterosexual marriage may seem to her like a lesbian relationship, which is what she wants. If my parent's life has been a lie, then my parent lied to me. Yes, and I bet it hurt your parent to have to do that. Perhaps that is one reason your parent has reached the point where they must deal with this.