Yes i read it. So? There's missing information:
Is he playing as France or was the king AI? There's not enough information. Hence my comment about the HRE war which suggests he's playing as France. If in fact he's controlling France there has NEVER, EVER, been any restriction against any player-controlled 16 year olds from marrying octogenarian spouses even in CK2, so I don't see what's the point.
My other comment is more on realism. So what if the MILF was infertile? If she's hot she's hot, i don't think youngsters going at it with cougars was a modern invention at all. Clergy aside, historical views on sexual relationships from ancient Rome to medieval times were not what most people today think.
Additionally, if this in fact happened to an AI French King the rest of my comment was that we need not model it after CK2 which is a very bandaid fix. Marry her and press her claims (Read: Her claims, not your children's claims. Not saying its probable, but a child/female liege can certainly happen in Agnatic-Cognatic.) or secure alliances, legitimize some bastards if he needed an heir.
I don't know, i feel like the OP sounded like he's new to CK franchise but I see he owns several CK2 expansions so I'm genuinely confused what's the gap here.
Maybe the OP finds it immersion breaking.
The general aim is to establish a dynasty
That's not going to happen when you marry a woman with 0 SMV
And bastards aren't a reliably viable alternative
*Gasp, but that's so politically incorrect and misogynistic*
Modern concept's and political constructs can FRO
It's a game set in the mediaeval period and in that period a women had 2 things to offer to a man
1 Her womb
2 Her feudal title claims
That was it. Sure it sucked to be a woman back then, but by modern standards it sucked to be a man as well.
The marriage acceptance scripts need some tweaking.
Women with a title and no heirs should only be accepting "matrilineal" offers
Men with no heirs should be looking for women with a high SMV
Young men with dynastic heirs should still prefer women with a positive SMV
All that appears to be happening at the moment is marriages are agreed solely by either prestige value or alliance
From the amount of inter-dynastic marriages I'm seeing in my play-throughs, I suspect Prestige Gain is the driving factor in marriage choices
That needs to change, because it's breaking the immersion.