I was really dumbfounded when Jeff Mirus said:
In this case, the continuing sins involved in the irregular union on the part of the repentant spouse would seem to be venial—on the grounds that full consent of the will to the moral evil of continued sexual relations is lacking. The sins would be rendered venial by either a very real confusion about the best course or the compulsion inherent in the particular situation, or both.’
So if your husband wants you to break one of the ten commandments, you're only committing venial sin because you're breaking them to keep the relationship.
What if your husband wants you to help him embezzle funds from his employer?
What if he wants to abort your child?
Rorate covers the theological misunderstandings much better than I can.
In this scenario the possibility of confusion about the best course is excluded ex hypothesi, since it is stipulated that the person in question recognises the sinfulness of the relationship. The question is thus whether full consent of the will to sexual relations is lacking. But this full consent is clearly stated to occur. The consent is the whole point of the scenario. The person in question is described as continuing sexual relations in order to prevent their partner in adultery from leaving. Acting for that reason means knowing what you are doing – continuing sexual relations – and choosing to do it in order to obtain a goal – the continued presence of the partner in adultery. Knowing what you are doing and choosing to do it in order to obtain a desired goal is what fully voluntary action consists in. There is no exculpating infringement of the will or lack of voluntary consent in the scenario. There is a fully voluntary choice to do a wrong action, made because doing the action has a result that the agent wants to obtain. Making choices of this kind is what sin is.
This was the very first lesson on sin God provided to us.
Here's what happened when Adam tried to persuade God his wife pressured him into it:
The man said, “It’s the fault of the woman you put here with me. She gave me some fruit from the tree. And I ate it.”
13 Then the Lord God said to the woman, “What have you done?”
The woman said, “The serpent tricked me. That’s why I ate the fruit.”
17 The Lord God said to Adam, “You listened to your wife’s suggestion. You ate fruit from the tree I warned you about. I said, ‘You must not eat its fruit.’
“So I am putting a curse on the ground because of what you did.
All the days of your life you will have to work hard.
It will be painful for you to get food from the ground. 18 You will eat plants from the field,
even though the ground produces thorns and prickly weeds.
19 You will have to work hard and sweat a lot
to produce the food you eat.
You were made out of the ground.
You will return to it when you die.
You are dust,
and you will return to dust.
Adam was tossed out of Paradise with Eve.
That's all we ever need to know about whether caving to pressure from a loved one closes the door on our salvation.