Someone can take a huge, unreasoned gamble and win. The fact that they've won doesn't translate into their decision being well reasoned, or negating of the possible fact that normally, taking such a gamble is more likely to be bad for them. The fact that they've chose to do it doesn't grant it any reasonableness either, in my view. It just grants it a level of respect as a personal choice, that might in some cases work out for people.
There is an element of personal preferences worth considering, but before the decision is considered to be valid or reasonable (at least by the person him/herself), i think naturally the reasons for the requirement need to be considered, putting in mind the glaring risk. Speaking strictly from a personal experience, the reasonings i've run into that make someone think that it's a good choice to specifically wait until after marriage pretty much have always amounted to some form or another of a flawed reasoning that ignores an unjustified leap. A leap from committed relationships being considered optimal, to failure at a committed relationship being considered a disaster.
Meaning, when i saw someone justifying their choice for waiting until after marriage to have sex, it seemed to me that what they were talking about was a desire to be in a committed relationship, but somehow there was a leap from that, to it being specifically marriage that must occur, and that in my view is of course an unjustified leap. Because if it was just the desire to be more sure of the person's suitability and the relationship's chances of success, why not have sex starting from the point where a marriage is being planned for? Or from the point where the option is thrown out there and both parties seem interested in it? Or even earlier when the relationship is both committed and successful for a while? It seems clear to me that it's because after marriage, people have more difficulty ending a relationship, and this is what the idea seems to be revolving around. I'm specifically speaking about when such an idea is proposed to have huge significance, which in my view relates to some religions (that propose that view) trying to deal with a subject perceived as difficult and dangerous (which it might've been so at some points in time, but is definitely no where near as so today), and where they've supplemented the above with teachings like viewing divorce as somehow negative, or outright forbidden. But in people's minds, in my perception, and in more cases than not, what happens is just that they adhere to family/cultural norms with a flawed reasoning rather than them actually trying to specifically employ what i explained above.
All that said, my personal experience is limited, of course. So i'm always open to learning and to hearing why someone thinks that waiting until after marriage is a good choice, despite of my view of it as an unreasonable choice. More specifically, if they see it as the kind of good choice that they would encourage their children to adopt, for example.