k

So what acts are forbidden?

The biggest one is anal sex. This is explictly forbidden in the Torah (the Rambam's leniency is that if people have been tainted by the desire for such, they should experience it once to satisfy that desire and then NOT repeat the act. However, he does make it clear that it is definitely prefereable to never engage in such an act).

The next major one with BDSM would be feminisation- a man may not wear a woman's clothing- once again this is an explicit Torah prohibition.

One area of BDSM which even Rambam would not allow is that of cuckolding- for obvious reasons! (For those who don't know what this is, it involves the woman having sex with another man with the husband/boyfriend present).

The prohibition against having sex with the lights on is easily avoided (just have bright lights on outside the room)- and even if people want direct light there are other poskim aside from Rambam that are lenient on this one.

Another area of concern might be that of male on female oral sex. Here there is a quite a debate amongst the poskim- the consensus opinion is that it is not allowed, however there is a large minority which many rely on that states that if is for the pleasure of the woman then it is allowed.

Overall, most BDSM activities would be allowed.

note: We do NOT Pasken according to Rambam. While the Mishneh Torah is an extremely important work, it is not authoritative and is not used to decide the Halakha. In that regard we use the Shulkhan Aruch and Arba'ah HaTurim.