Walkthrough guide link is the front page:
https://attachments.f95zone.to/2023/10/3004918_Mini_Guide_up_to_v.0.17.5.pdf
There you can find:
0m. Secret - In BDSM don't ask about red syringe. When you have sex with her choose option to inject her with the red
drug. If you give the gang to Madam and Secret is pregnant you can invite her to your house. You can have up to 2 kids
And you asked what is the connection : You made Madam your enemy and Secret is working for her... so BDSM club is closed for you and everything and everyone inside is locked.