How much of the 'dread' was because it was painful and how much of it was just a consequence of homophobia?

I imagine it's neither. It is the psychological aspect of the violation. Just as why many women dread vaginal exams, or even having their breasts examined. Necessary but clearly undesirable. I think it may actually be more violative for the average gay man compared to the average straight man, since it's probably more closely tied up with his concept of intimacy.

I was thinking more in terms of it coming from the same place as dudes who refuse to wash their butt because it's 'gay'.

Mammograms hurt a like shit for a lot of women. People aren't just upset about this because of the violation. There is real actual physical intense pain associated with these procedures.

This is to say nothing about various gynecological exams. "Let me scrape your cervix with a metal tool" is a pretty common procedure, for example.

There are non invasive prostrate cancer screenings with a urine test now.

Also.. your reply informs me that you have no idea what a mammogram procedure involves. And that there are other non invasive methods that insurance won’t cover. And that for most women, the first screening is almost always inconclusive leading to a second one. Multiple unnecessary painful screenings because it’s ‘cheaper’ but also a standard procedure.

Digital rectal examination is largely dreaded because it is "gay" to have something inserted in your anus.

When my wife had her IUD inserted, it was the most painful thing she had ever experienced by a long shot, more painful than multiple invasive surgeries and debilitating back pain. The doctors told her that it would feel like "a pinch" and refused to prescribe pain medicine.

I don't think this is comparable.