I think there are 2 issues coursing through your post, which are kind of interesting to discuss, one is exclusivity vs. non-exclusivity in arrangements, and the other is either the SD getting sick of the SB, or the SB getting sick of the SD. These are related, but distinct issues for reasons that I will discuss based on my own experience.
I have almost 10 years of arrangement experience, mostly on SA, but I dabbled in some other dating sites as well. Initially, I thought I would want to have an exclusive, long term arrangement. Over time, however, I learned that is not what I really wanted. First of all, I realized that most SBs did not want an exclusive arrangement, many had BFs, and some just did not want to be "committed" to any one person whether a SD or otherwise, and believed in open relationships. In addition to that, I discovered that some SBs I got sick of pretty quickly. Even the ones where I continued to be strongly sexually attracted to them, I still wanted to have other sexual partners besides them.
I was in a 3.5 year arrangement from June 2017 to December 2020 with one SB. She was great. Initially with her the sexual part was restrictive, but over time it became less restrictive to the point where it was not restrictive at all. There was genuine mutual attraction. We went on numerous trips, the final of which was to Maine at the height of Covid in August 2020. It was one of the best trips in my life- maybe the best. The lady had an off the charts energy level that was contagious. She wanted to do everything there was to do. We went to the beach, we went to wineries for wine tastings, we went on a harbor cruise, we did culinary tours, everything you can think of and more. The first two days of the trip I never felt more fatigued in my life, and it was also partly because she could drink like a fish and not only not lose energy, but gain it. She was a human charging cord. In October of 2020, at a time when she was 26 years old, she told me she wanted to bag the arrangement and instead have a serious relationship. At that point I had only been paying her for intimate dates, so I agreed. Shortly after agreeing, and after a few more dates, she told me she wanted a baby with me. She said she had "baby fever." This was shocking news as early in our arrangement, she had told me of her plan to foster a child and then adopt, and that she did not want to have to deal with childbirth and raising a baby and instead would adopt a 5 year old. This plan changed, in part I think because her first cousin in the prior year (2019) gave birth to twins. She then became extremely emotionally attached to the cousin's toddlers and was always showing me pics of them. Well, I was not prepared to be a father for the first time at my age, had no desire to have children, and told her that. Although I also told her that if she decided to have her own child I would be a father figure for the kid, but would not support the kid. I was, soon thereafter, history. I was quickly thrown out of her life.
Despite how well that arrangement had worked- it was great in all respects- I never had any desire for exclusivity with her. I dated many other women while seeing her. Initially that was because she was (at first) not so great sexually, but later it was because I just wanted to have sex with other women. I realized I was not monogamous.
In 2018, while still in the arrangement with the one noted above, I started seeing another woman who was slightly younger. Also met her through SA. The one I saw from 2017 to 2020 was a black lady, while the younger one i started seeing in 2018 is half African and half Portuguese, but looks like Ariana Grande. I still see her regularly over 5 years later, but our arrangement has evolved. Although initially I did dinner dates and even a trip to Maine with her, over time, I realized she is really not a dinner date or trips kind of SB. Her idea of where to go for a fun dinner is to get an egg sandwich on a bagel from Dunkin Donuts, washed down with a Coolata. Or a frappe from McDonald's. And on the one trip I did with her, she was not like the other SB I mentioned above. Not a great energy level, more of a person who had rather go to the movies or stay at the hotel and watch TV. Sexually, however, she is fantastic, and I never have tired of intimacy with her. In the 5 years I have been seeing her, she has cycled through 2 boyfriends. Both of those relationships ended very badly. Although sexually she is fantastic, and has been my primary SB since 2020, I still have a strong desire to meet and develop intimate relationships with other women. It's a sense of sexual curiosity that I have. It's as though I am eternally in a quest for great sex and having as much of it with as varied a number of women as possible, a la Wilt Chamberlain. I might not ever surpass Wilt, but I will probably keep trying.
I believe the guys who really want to have an exclusive SB are, at their core, needy and possessive creatures. Wanting to have a woman on a leash or rope or on speed dial is the only possible reason in my mind for an exclusive SB. I cannot grasp another plausible reason. From the SB point of view, there is a plausible reason for exclusivity, with one SD, which Alisha has already mentioned. FINANCIAL SECURITY. Indeed, all the ones who want an exclusive SD need a set amount per month, have limited availability for SB activities, and therefore one SD is the only way to make that security happen. So I see exclusivity as a one way street. And even the SB with an "exclusive" SD, she may and usually will have a BF.
since we had talked about our tastes in music (I always travel with a Sony mini speaker for such occasions). I was surprised when she accepted because it was late and she had to work the next day but when good fortune smiles on me, I don't don't question it.
