If a witless becomes pregnant (and is not married), that generally amounts to fornication. There may be exceptions, such as if someone is raped while incapacitated and the Filthful and Disgraceful Slavebugger has a current stand that it is not currently the victim's fault at the time of the incident).
To them, fornication appears to be the most serious sin one could commit (second, of course, to apostasy). I judge that from the amount of time and energy they devote to preventing people from having access to any opportunities to do it. If you are baptized, you will face a judicial committee and, if they decide you are not repentant, you will get disfellowshipped. If you are not baptized, it will hold you back from baptism. If you are an unbaptized publisher, you will lose that "privilege".
It really doesn't matter whether it is with a worldly person or another witless (though if another witless, there will be two people hauled up to the back room instead of just one). Marrying the person you did fornication with generally will not stop them from disfellowshipping you, and that is doubly so with a worldly person. You will be ordered to stop seeing each other (if you are both witlesses) to show "repentance". If it's a worldly person, you will have to give up all contact except getting them into the cancer to prove repentance.
To me, the whole business is hogwash. The only thing I can see about fornication is that, if you have a baby, it's your responsibility for the next 18 years (and in some countries, it's 21). And there is the risk of catching a disease (though that can be offset by reduced risk of other diseases if the sex stimulates your immune system). Other than that, they are really creating major problems where no problem should exist.