Good question! It's somehow on par with violating minors, prisoners or the cognitively impaired...by virtue of fairly recent laws. The public doesn't feel antipathy towards dual relationships, like they do about adults initiating intimacy with minors. If you had a relationship with your T, it might be viewed by your peers as odd, but not transgressive or abusive. But the psych profession and state laws sees things differently. They created this current paradigm. It's like the new and final frontier of transgression, replacing what used to be unique stigma of LGBT relationships. If you're interested, there is empirical research into negative effects of sexual intimacy between T and C. It's a hard topic to survey because of nonresponse bias, but the data suggest strongly this is a different kind of power differential than what you have outside therapy with a dominant partner. It causes real damage in most affected clients.