@alessiavassall: I read your question like actually two questions. The first one would be "Why are people trying to convince me of having my boyfriend involved?" And the second question would be "Why is anybody turned on by watching their wife/GF or being watched by their husband?"
The second question i can answer only from my perspective as a man with a somewhat voyeuristic mind. i have two very personal answers to give here. The first is that i have learned from experience that it turns me on. That would be the short answer because i could tell you my sexual life story and that would perhaps not only need its own thread but an own forum for that.
The second answer is that my pleasure is strongly related to my Woman's pleasure. Seeing her receive pleasure and having an orgasm is a major source of my own pleasure. There are advantages and disadvantages in both scenarios, where in the first i am the one giving pleasure to my Woman and in the second she is receiving pleasure from another man. Me having sex with her is great for the closeness and physical stimuli. Me watching her having sex is great for the visual aspect and the chance of getting to see her full body reaction to being manhandled and serviced well. An obvious example that explains the difference well would be her taken in the missionary position (but you can think of any position with no difference here). When i am the one who fucks her it is all hot because it is a very close position where we can feel almost all of each others bodies, we can kiss, whisper naughty somethings in each others ear, and so on. On the other hand, when she is fucked missionary by another man i get the chance of enjoying to see her full body reactions including curling toes and many other things i had no chance of seeing if i was face to face on top of her.
Anyway i never felt other guys having sex with my GF would encroach on our intimacy. We, both high libido persons, would have sex once to a few times a day while the arranged meetings with a third player were no more often than once or at most three times a month.
i can't answer the same question from a female perspective but i'll try: i have heard at least some Women say that it would turn them on to see their husband's reaction while being fucked by another man and they would have a lot of eye contact, kissing and holding hands - in other words: a lot of intimacy going on - with their hubbies in the process.
If that's not for you - fine! It is understood that not everybody has the same fantasies or desires.
Another aspect would be feeling safe and secure. Some Women are very concerned about going home with a stranger because you will never know what person that actually is and what is going to happen, and the Woman is typically the weaker, more vulnerable person and therefore more at risk. Having her partner there for comfort and protection just in case would ImO be another answer to your question.
To answer your first question: i believe that - no matter a member is single, in a relationship or just a fantasizing wannabe - most do not support cheating. Single Women may see themselves in a relationship one day, couples do typically not want their significant other to cheat on them and don't want to have to deal with the guilt if it is them who cheats on their partner. And the wannabes are not fantasizing about their (imaginary) wife or girlfriend going behind their backs which would cut them out when all they want is being part of the action in one or another way, which is their fantasy. With Black men it might be slightly different but also many of those do not want to be home wreckers. Also it is comforting to know not having to deal with a jealous boyfriend in case you are caught in the act.
- Just a few thoughts.
In the end you don't have to bother with all of that if it will remain just a fantasy for you.
But on the other hand it seems to be a very strong fantasy, perhaps even a deeply rooted desire of yours? (Considering you have joined a site that is themed in line with your fantasy, and considering you obviously spend some - perhaps a lot of - time with that fantasy.) Once that fantasy or desire will get the better of you you will have to make a decision. And ImO, if that happens, there are only three possible solutions: (1) leave your boyfriend, (2) cheat on him, or (3) involve him. Honestly i am - like many others - also not a supporter of the cheating scenario. So my "advice" would be either (1) or, if you value your current relatioinship high enough and don't want to lose your boyfriend, then (3). - Just my almost worthless two cents added.