Thank you for reaching out to the Wefugees Community.
Marrying as an rejected asylum seeker in Germany is not impossible, but also not easy. You will need to hand in many documents. Also, your only option might be to marry here, as you can not travel/leave the country and come back as a Duldung holder.
The topic of marriage was discussed many times already on this platform. Please see, for example, this thread for more information. You may also want to check our blog, in which the required documents are listed.
Regarding your passport at the BAMF, you could try to ask them to give you a legally attested copy of it.
At the end, the authorities will always try to make sure, that your marriage is also not a "fake" marriage.
I hope this helps and good luck,