Because everyone has personal preferences and mine is that if I've gone for almost a quarter century without kissing a girl, I would like to marry someone who also abstained from such acts before marriage. Repentance is not relevant because the issue is not about the level of a person's iman. Between two choices, I would choose someone who's less religious (assuming, of course, that the basics are met) who's kept herself away from zina than someone who's more religious now but did zina in the past and repented.

Honestly, I feel it's more than a little unfair to the sisters who have abstained from zina and relationships that we discount it completely and say that we have no preference as long as the person has repented. This post is relevant. I know others don't feel the same and that's their right. But for me, it's a hard requirement and something I'm not planning on compromising on. If I were to find out after the marriage had taken place that she had lied, divorce would be almost certain (well, technically speaking, it would be an annulment since the marriage ends but the wife has to give back the mahr unlike in divorce where the wife keeps the mahr).

As to how to know if a person has done zina, well, there's really no other way than them telling you and you asking. The way I plan on phrasing it when I'm in the situation is by stating the above and leaving it at that. That way, she can find an excuse to turn me down without having to admit to doing zina in the past. With that said, if a woman asked me point blank, I would not be offended and would respond truthfully.

I'm not sure what you mean by the woman stands unfairly in this position. The only way to find out for men and women is the same.