There are many aspects of the haredi (ultra-Orthodox, as opposed to Modern Orthodox) baal teshuvahs experience that aren’t properly understood by the recruits of the movement. One of the many considerations that need to be understood better and earlier in the teshuvah process is the status of ben niddah, or child of impure menstrual blood. This is a child born to a woman who had not immersed herself in the mikvah, or ritual bath, prior to sexual relations, as is commanded by Jewish law.
Of course, the vast majority of liberal and secular Jewish women do not go to the mikvah prior to resuming sexual relations, and therefore most children of liberal and secular Jewish backgrounds are designated as “b’nai niddah.” In fact, the term BT (baal teshuvah) is today essentially synonymous with “ben niddah,” and this may be why the term baal teshuvah is employed more frequently in the haredi world for designating a newly observant member of the community than in the Modern Orthodox world. A BT does not just come from a different background as a haredi FFB (“frum,” or observant, from birth), but is also of a different status than an FFB. This is because there are negative personality characteristics associated with such a classification according to many ancient rabbinical commentaries.
Bnai Niddah are “corrupt and sinners.” They have a genetic disposition to do evil. They are prone to brazenness and rebelliousness, and do not treat great rabbis with the proper respect they deserve. Baal teshuvahs are not properly deferential towards great rabbis just because they were brought up with and retain vestiges of a liberal democratic approach to life and society. It is because their mother did not immerse in the mikvah, or at least, the BT’s unfortunate world view is exacerbated by the unclean bloodstains of menstruation on their souls.
These are considerations for haredim to not only refrain from marrying BTs, but also to refrain from marrying their children, as such a pagam (defect) is considered to be hereditary. Among other issues, there is enough of a discrimination problem that Beyond BT, a support website for established baal teshuvahs, openly discusses whether or not a baal teshuvah should hide his liberal/secular background.
So how does the haredi world continue to gain BTs who must accept his/her defective position in this quasi-caste system?
First of all, the b’nai niddah issue is delayed in terms of its explanation and understanding to haredi recruits. A BT is usually already well on his way into haredism before his status as a ben niddah is revealed to him. At that time, dismissive disclaimers from the “gedolim” about the status of b’nai niddah that are offered to the Baal Teshuvah to soften the blow. These disclaimers are offered every time the issue of ben niddah is brought up.
But are they truly dismissive?
As Mayim Rabim notes,

The majority of gedolim in my circle have dismissed the ben niddah concern nowadays. But the reasons they have come up with for doing so seem so strained. To paraphrase some examples from the same article:
The Steipler Gaon: The concern regarding a ben niddah’s character is merely statistical. If an individual shows good character, he is obviously an exception and the warning can be ignored.
Another opinion cited by the Steipler Gaon: The blemish of ben niddah is hereditary for an infinite number of generations, not just one, and in fact all of us are likely to have it (or some other blemish) somewhere back in our lineage. So we’re all on equal ground and have no reason not to marry each other.
Rav Moshe Feinstein: In many cases we can’t be certain the mother was truly a niddah mide’oraita, because maybe she went swimming after her period in a body of water that qualifies as a mikvah, and thereby became tehorah. (Rav Moshe does not discuss the fact that she would most likely have been wearing a tight-fitting bathing suit at the time.)

But how many exceptions are there? And how many women happen to go skinny dipping at just the right time? And are most haredim really willing to claim that somewhere along their yichus (lineage) something probably went wrong somewhere? Publicly, through marriage of a child to a ben/bas niddah? Even according to many who feel this is an issue that can be worked through, each subsequent generation will have to do so. Ben Niddah is the gift that keeps on giving.
So for the masses of haredim, the status of a “ben niddah” is a reality, even if it is not quite halachically mandated in terms of prohibition of union. They may be technically allowed to marry baal teshuvahs, but it is hardly advised. The relatively left-wing ultra-Orthodox and candid Rabbi Homnick once told a group of us how a child whose mother went to the mikvah was much more desirable to haredim even in terms of adoption.
It appears that The Gedolim (rabbinical leaders of the non-Chassidic haredi world) are not really dismissing the status of b’nai niddah, but only downplaying its role in the public discourse, which since the haredim are heavily in the recruiting business, makes a lot of sense strategically. A separate, lower class of Jew is still being created, but the lines are blurred just enough that its exact role and meaning can be obscured if one wants to see it through such a myopic lens, both in terms of the baal teshuvas themselves, and for normative haredim who seek marriage with them or their offspring (the latter situation arises more frequently than the former).
It is important that the issue of ben niddah is understood by liberal and secular Jews and their families when they are entering or considering entering haredism, not after the fact. They need to understand that in the eyes of many, they are not only second class citizens (forever) because they grew up in a secular or liberal environment, but that this status is justified by the circumstances of their creation, and should not be assuaged by the facile non-dismissals of the status of the baal teshuvah/ben niddah by select haredi leaders which many kiruv professionals, most of whom are haredi, will offer when pressed on the issue.
These are the best responses they have to offer.