I haven't played Fable but I believe that game allows romance and marriage how does it work there.
If it were done right, then I can't think of a better addition to the Fallout experience- to come home to a loving spouse after a hard day's (or sometimes many days') work. However, here are some major pitfalls to avoid:
Your pit falls are mostly woefully unrealistic to ridiculous. Especially your hang up on psychologial problems that don't always occur in real life and have yet to ever occur for the main character in a fallout game. (unless the player roleplays them as a psycho). Many of your points kind of repeat themselves to.
1.) Age barriers- the Lone Wanderer is only 19 when he steps outside the Vault. How on earth will he be capable of sustaining a romantic relationship when the Sugar Mama angle is present?
If 19 is old enough to become a super human warrior and save the wasteland its old enough for that. It can be implied that a male LW and Amata held romantic feelings for one another.
And Sarah makes a similar comment of attraction towards a male LW.
2.) Unstable conditions- you work night and day in the wasteland, your spouse will never know when you'll come home, or if you'll come home. How could you expect him/her to be able to deal with that kind of uncertanty?
People did it all the time for centuries, If someones profession was a sailor, hunter, trapper and a few dozen others they could be gone for weeks or months at a time. I'd imagine in the fallout universe there would be plenty of wives waiting days or weeks for there husband to come home from scauaging or a trading expedition.
3.) Psychological barriers- let's face it, the protagonists of this series haven't been exactly the most level-headed men. The nightmares, the flashbacks, the sounds of the dying, you'd have to a pretty messed up person to just accept that as the order of the day. PTSD would take a huge toll.
So soliders in real life shouldn't be allowed to have families? really how do you think every other wastelander reproduces. There are plenty of scavangers, mercenaries, slavers and radiers living in the wasteland and I presume reproducing. The hero must be resistant to PTSD because the game doesn't force you to flip out and shoot someone when a guy sneaks up behind you on the Strip or other such safe community.
4.) Money- how will the player support his family with only the money he/she makes from selling scavenged weapons? Think of how expensive it would be to feed yourself and your spouse; you'd barely have enough left over to buy a box of 10mm. How will the player manage finances? What about taxes and bills? Will it be like the Sims where a repo man comes and snatches your gear if you don't pay up?
Are you *&)*$@ kidding me? you can become extreemly wealthy scauaging weapons and equipment. If the wasteland scavangers support a family the super human main character could certainly do the same. The wealth that pours into the main characters hands could allow them to build an entire wasteland community and still have money to spare on all the weapons and ammo they need.(I'd like a battlehorn castle style DLC for New Vegas quite frankly.)
No one ever comes to repo your stuff in Megaton why would it suddenly be an issue if the Lone Wanderer could have married Lucy West? If there is any tax system it must be a sales tax or an import tax on caravans.
5.) The spouse's feelings. Think about it from the spouse's point of view; he/she (the spouse) will marry someone who will hardly ever be home (you), and only comes home to drop off their stuff and hit the sack. How will he/she spend quality time with them? What about the inevitable questions "Have you been faithful?" and "Will you ever stop wandering" or the classic "Why aren't you ever home" or the dreaded "Can you babysit junior while I go shopping"? How will romance balance against the fact that the focus of Fallout is combat and the thrill of the chase?
The timeline of a fallout game is to short to include anything like children unless theres is a timeskip. As I said before in centuries past it was common for a husband's job to keep them from home for weeks or months at a time. And in the post war world, scavangers, mercs and trades would be gone for similar lengthy stretches but many of them would have families back home.
Additionally it would be sensible that your girl/boy/friend or wife/husband would live in your home base. Most Fallout 3/NV players frequently return to a single home to store gear for a later time. So spending quality time isn't that big an issue. Having dinner with your spouse for instance could simply be a fade to black which then grants a well-fed condition. +XDT for X number of hours. Maybe a few comments about your actions on your last quest. If the house is in a town then a simple date can be accomplished by going to the local restaurant.
A high level of detail is unnecessary just an implication at what happened can be enough and time passing on the clock.
If the next fallout were to feature a dynamic, rewarding and fun romance system, (with adjustable raunchiness) then the player character would need to be capable of supporting a romantic relationship. This would require someone with a superhuman ability to tolerate extreme gore on a daily basis who is still able behave like it is something horrible and alien when with others. This doesn't make any sense, and would require psychological modelling that would be more annoying than the Vault 13 canteen. For instance, you would seize up after cutting a Raider in half with an auto-ax. Sleep would feature nightmares and your character could wake up crying in the middle of the night.
I guess I'll take a response from one of your previous points. These psychological problems your imaging don't cause the main character to suddenly flip out when he's walking through megaton or the strip so there's no basis for it. If what you said was true the main character of the game would be experening those things without a romantic partner. Seeing as how none of what you say actually happens in the game there's no reason to add it just because your dating someone.
It wouldn't be worth it in the end. Having to navigate an ocean of dialogue options just to get through the day makes the game too much like real life.
Why would it be done like that, plenty of video games through out history have been able to have the main character become a friend or even a romantic partner with other characters even if that character isn't in the party. Unless the RPG game follows a liniar story with set characters(including the main one) the romance is always optional. In Baldur's Gate 2 and other bioware games the romance is optional. In Final Fantasy 8 the romance between Squall and Rinoa is not optional its part of the story. As Fallout is more like the former that would be where romance would fall if it was included.
In the end, romance in Fallout would require two people who can show deep love while not having that which sustains these relationships- stability, maturity, and feasibility. The marriage would fail and both parties would end up brokenhearted and dissappointed. Unless the developers will allow you to have your spouse as a follower (highly unlikely due to the fact that they would quickly end up dead or breaking the game),
Lets say you could romance Cassidy or Veronica, the followers are rather durable so they'd survive just fine. *especially if your not playing on hardcoe*. Stages of the romance could easily be accomplished like stages in a quest such as each followers personal quest. Your standards and expected level of detail are ridiculously high.
And as I've already explained there are plenty of people within the world of fallout who make a living in the wastes, and they would have families back home. Its not the 20th century, socially its more wild west or medieval. It be considered normal to have a spouse whose gone for long periods the line of thinking would be quite different. The real worry would be your spouse is only with you because you can provide such wealth from your occupation as super human of the waste.