Most colonics I have had the therapist inserted the speculum. A few years ago, in Oregon I was told the law did not permit the therapist to do the insertion, so I went to another who never asked, just did the insertion.
I saw mention of the open system, what a waste. I inserted the tube and was left by myself for about 40 minutes, only plus side, I could have masturbated if I wanted too!
My best insertions have been by therapists that do a rectal exam and lubrication first. I've had a couple that I'm sure knew darn well it was erotic, and made the process very favourable.
A few years ago I was helping a young woman set up a colonic clinic, and since I had experience as a therapist and owner I helped her get started. I suggested she be on the receiving end for lesson one, and took the opportunity to spend a lot of lime on the lube and insertion explaining that you wanted your client relaxed and comfortable, and that the exam and lube went a long way in relaxing the client.
When it was my turn I encouraged her to explore, so she would know what to expect.
She had me on hand for the first few clients, and one was a very lovely woman in her early 30's. I explained to her I was there to assist and train, and both of us spent a bit of time on examination and insertion, which the client was not only comfortable but seemed to enjoy.
This I was training woman is a lesbian, and planned to do colonics for the gay and lesbian community.
I will close the commentary by saying gay folks of either sex are generally very accepting of penetration and generally find it enjoyable, and have no issue with people who's sexuality includes enemas.