I wonder if the fact that Louis didn't marry Tessy as soon as she became pregnant wasn't a way to see if she was just a golddigger.
I mean Louis had to know how unrealistic it is that the succession would ever be his responsibility. But apart from dynastic reasons, which didn't exist in his case, there was no reason to marry nowadays only because of a pregnancy. After all his family has recognized the child, even though it does not have a title. But as can be seen with the Behn children in Norway, there is no need for a title if the family loves the children.
OTOH it makes sense not to marry if there are signs that point to a fast divorce. But after spending the time of the pregnancy and the difficult early days as parents together, I guess both Louis and Tessy have gotten a better or more realistic feeling for the responsibilities and chances of their relationship. And surely Tessy would have shown her true fetahers if she had indeed played for the title of princess or for a quick and financially interesting divorce on becoming pregnant. Obviously, she didn't. She has stayed with Louis and they've shared their responsibilities. Thus, I believe, they decided to marry - for they surely believe that their marriage has a chance. Just my opinion, of course.