Not all women are cold sleepers. Not all men are warm sleepers. For those of us who are cold sleepers, the "comfort" level will be about 9*F higher, as mentioned.
I looked up (on a UK website) the EN13537 ratings on my Western Mountaineering Ultralite, advertised as a 20*F sleeping bag. I don't know why WM doesn't have these on their website, since they do sell their bags in Europe. "Lower limit" (men/warm sleepers) was about 16*F; "comfort" (women/cold sleepers) was about 25*F. Interestingly, for me the "comfort" level is where I have to start adding extra warm clothing; the "lower limit" is where I start getting cold even with all my warm clothing on.
At least those who say the WM ratings are very conservative are correct!
Re liners: I tried a Cocoon brand silk liner some years back. I repeatedly woke up so entangled in it up in it that it took me several minutes to get out of the sleeping bag. After a few nights of that nonsense, I ditched the liner. As far as I could tell, the liner added no appreciable warmth. I use my base layer and a fleece hat to sleep in,and those stay relatively clean. The base layer and hat also keep me warm (under my hiking clothes) on cold mornings and evenings, so they have more uses than the @#$!%^&! liner!