I can understand how that would be abusive in some cases. But it doesn’t match what I see. A lot of time people in these relationships engage in more like what straight people call “swinging”, and there may not be any significant age gap. Even when there is an age gap, it usually doesn’t seem abusive to me. A lot of the times the younger partner is more outgoing, is actually the main person coordinating socialization and in that sense has a lot of agency. But there are endless configurations and it is hard to generalize.
When I have seen abuse, it’s usually either someone who is controlling of their (only or primary) partner, or a single person who acts predatory (maybe too creepy to get in any deep relationship). A “third” or fling usually would have much less tying them to the relationship and can more easily cut and run if things aren’t fun.
Then again I don’t want to deny the existence of abuse just because I haven’t seen it, people cover stuff up and it’s good to speak out.
FYI your account is in a semi-banned state. This comment was marked as dead and I could only see it because I have “showdead” setting turned on. I vouched for this specific comment so it might be visible now.