Is this ok or just a bad idea?

Being involved with your boss and starting to get involved with a coworker at the same time sounds like a bad idea all around to me.

I really have no opinion on how it will or will not affect your relationship because that's between you and your boyfriend. I would take the fact that he doesn't care as a giant red flag if it were me, but that's your call.

However, if that happens at work and you respond in a way he doesn't like, he can actually sue you. If another coworker sees it, you both can be sued. That is severely inappropriate to take place in a workplace. It doesn't take much to hold up as a sexual harassment suit and your being involved with the boss makes it even more plausible for someone to get litigious and sue based on trying to gain managerial favor through you. It could go wrong in more ways that I can count, and what would 'going right' even be? If it could cost you your job (and honestly if I was the boss and saw or heard of someone getting spanked at work, the spanker and spankee would have already been fired), is it really worth it? You constantly comment that there are no other jobs available to you and your health puts you at risk for losing this one, why put yourself at a very real risk of being fired or even taken to court for a cheap thrill?

if you don't report getting spanked to your superiors, then yes you can fired. If it's not reported, then you're indirectly stating that that kind of action is ok between you and him. If an inquiry occurs, what he could do is that he could say that you never complained so you must have liked it and thus permitted it. Most bosses will assume "Since she didn't say anything to me about it, then it must be OK", even if you don't personally feel this way. Most places of work (unless you work in the sex industry) do NOT permit that kind of sexual display.

Since I know you were devestated that you and your boyfriend broke up a few weeks ago, I would come clean about what's going on between you and your other coworker. It will look a lot better than him finding out about what you guys are talking about and doing. However, if you're in the type of relationship where that kind of behavior is permissible, then you should have little to nothing to worry about.

If anything, what you need to do is to switch places, so to speak. Imagine you are your boyfriend and your boyfriend is in your shoes. Would you be okay with a girl talking about getting spanked with your man? And would you be okay with a girl slapping your man's ass? If the answer is no to these questions, you better talk with your man AND your coworker. Honestly, just letting him spank you is going into something else. Stop it before it starts. If you care about your relationship, it would be good idea to stop touching and keep conversation ONLY work related.

If this coworker knows you're already in a committed relationship, but disrespects your relationship, then you shouldn't waste your time with him. That's straight up disrespect to both of you. And someone that can't respect you before a relationship won't consider respecting you during a relationship. Tell him to stop because you can both lose your jobs, and it's not right to your relationship (that is assuming that this does violate the boundaries in your relationship). If your coworker still doesn't stop, then you'll need to file sexual harassment claim.

Anyway, I'm hoping you take everyone's advise here because in this economy it would suck to lose your job. Getting emotionally invested with coworkers is tricky, but I hope it works out.