It probably has to do with the difference between "heir presumptive" and "heir apparent". Caitrin's position could be displaced by the birth of Roger's child, and by the time she married, Roger was already heavily involved with Angelique. So though Caitrin was the heir to the Manticoran throne, everyone knew she probably wouldn't stay that way.
My response:
According to "I Will Build My House of Steel", Roger and Angelique and Ed Henke and Caitrin Winton had a double wedding...
So I guess Roger was heavily involved with Angelique when Caitrin married Ed!

Don