Hi. I have actually experienced something VERY VERY similar, except from the other side. My guy's parents are conservative christians (so are mine, but they realize that at 21, I get to make my OWN decisions). They lecture both of us all the time about how "Jesus is watching" and such...

My advice is just to ignore it. They are not trying to be mean or controlling, they're just trying to avoid you growing up. Be as understanding as you can be, but, this is a big one, move out as soon as you can. Part of the reason your mom probably feels you are not mature enough to be on your own is because you don't stand up for yourself and say you will. I realize that moving out on your own is hard (both Me and the guy had to do that), but it is well worth it.

Also, I agree that coming out about the BDSM nature of your relationship is not a very good idea. I have come clean with my parents, but I also have a very close relationship with them... and at this point, it would probably conjure up images of you being abused, which would definitely not help the situation. Don't LIE to them, but just don't talk about it.