[QUOTE=featherlou]
Careful with your quotes there, Vihaga - I was quoting from a book; those are not my words.
I see your point, that is it not possible to tell from an fMRI that you are rationally incapacitated, but there is much more evidence that people in the throes of infatuation are not their normal selves. Taking your point into account, I’m not comfortable completely disregarding the conclusion of the authors of my quote that people are not using their rational brain when they are infatuated; it does explain a lot of spectacularly bad decisions. 
[/QUOTE]
Sorry; I pasted and didn’t realize the tags didn’t go through. I meant the quote to be in a separate indent from your note. My point was merely that that particular piece of data tells you nothing about an individual’s ability to make rational decisions; your authors presumably could’ve done a lot better in supporting their point.
/slight hijack
I actually saw a really good talk about this (not specifically, but more generalized) yesterday, from a guy named Pete Redgrave, who studies the computational function of different parts of the brain. The talk was about determining the function of the basal ganglia, and after considering the constraints imposed by biology (both evolutionary and structurally, as it is a very old and highly conserved structure) and consistency with their models, his theory was that the purpose of the basal ganglia was selecting what stimulus/processes to act on. The idea is that the entire structure consists of a bunch of parallel circuits that integrate competing signals, then choose one to act on. (You only have one body, so at any moment you have to choose between food, drinking, sex, or whatever, depending on the strength of the motivation.)
What’s interesting about this is that it’s a problem almost all animals have to solve, whether they’re lampreys or people, and the connections indicate that there may be this same competition between the rational (cortical) and more “base” (subcortical) structures- and that the one that wins is the one that shouts the loudest. The point (I do have one) throws free will out the window a bit, but is interesting to me, as it provides a neat perspective on how people make different decisions in a given situation. It’s not that someone lacks willpower, necessarily, but what if his drive for sex is just screaming compared to someone else’s? That idea also explains a lot of “bad” decision-making; we all do things that we know aren’t the best idea occasionally.
Sorry about the hijack, my striatum compelled me to share! 
/end hijack
As far as the OP goes, this is really a decision that is going to be so couple-specific that there’s probably no good general rule of thumb. Everyone has a different comfort level, level of self-trust, and level of partner trust.
it’s also none of your mom’s business. My mom thinks it’s weird that one of my friends (F1)is comfortable with another friend of hers (F2) staying in their house, considering that F2 once had a relationship with F1’s husband years ago. I didn’t even think about it being weird. Everyone’s mileage varies.
/edited for horrible grammar and typing, some of which persists