suviko,
Since you responded so nicely to my comments and shamed me for not answering the questions, I feel obligated to speculate.
First of all, I am not going to say an automatic "NO" to any of them. Any relationship, whether dating or marriage, is based on mutual attraction, mutual interests, and trust.
I would have some problems with "sharing" someone I care about, but the question then becomes one of how important the relationship is to me. If the relationship is important enough to commit myself to that person, we will find a way to work it out.
The issue of STDs and the potential of problem "fans" is another level of concern. Assuming that our relationship is based on the criteria I outlined, then we can work out those issues out too.
To me the question goes beyond the moral dilemma that those professions cause. The question is whether you are willing to see the person instead of the profession. I think that I am open-minded enough to do that.
But this is all speculation. I am not likely to meet anyone who is in any of these professions and I know that my wife is not willing to share me.