Really why is anyone in a relationship? Its to enhance the quality of our lives. It also doesn't mean the same things for everyone. Love, companionship, material gain, etc. Throughout history marriages were based more on financial gain and continuing the family line then personal feelings. You did what you had to survive.
In today's world, not everyone is suited for a monogamous relationship, and I know a few very sweet guys who entered into SD/SB relationships because they wanted companionship but due to their jobs (long hours, lots of travel, etc.) they knew they wouldn't be able to give a woman a regular relationship. And I think in some SD/SB relationships and even in some instances of prostitution, the woman is really the one in control. Personally, I was approached by a guy for a SD/SB relationship. I ended up not taking it because I moved because of my job, but it wasn't as dirty and underground as some people are making it seem. He and I drew up a written contract detailing all aspects of our relationship. He was based in Australia, but traveled to the states every few weeks or so. He didn't want to stay in a hotel by himself every time, so the arrangement was that he would provide me with an apartment of my choosing, furnish it, etc. He also included stipends for a car, gas, food, even pets if I wanted them. He requested we share a bedroom, but was specific that I was under no obligation to provide any sexual favors. Basically he wanted a girl to 'come home to' while he was traveling to the states so it wouldn't be so lonely for him. He was a decent guy, and even gave me the phone number of a girl he had had a similar relationship with previously if I had any questions; she had been a college student who was estranged from her family and was just making ends meet. She was very sweet and only had good things to say about the guy.
I did watch a documentary recently about underage girls being forced into prostitution, which is a completely different scenario that I don't agree with, but then on the other hand there's places like the Bunny Ranch. Would I ever work at the Bunny Ranch? Probably not, but I don't see those women as 'dirty' because they found something they enjoy and are able to support themselves with it. Why is it so shameful to use our assets? What's the difference between using our brains and our bodies? Maybe I just don't see this issue as so 'taboo' as others, but I respect women in control of their lives. And some of these women are doing exactly that.