Ha! Buggering!
As for myself, I’d introduced anal play into my own masturbation since I was about 14, so the first time I decided to be the receptive partner with a guy, it wasn’t that difficult- just a little lube, and it was fine, and his dick wasn’t exactly intimidating. My partner tells me I’m atypical though.
For my current practices, I like to warm my girl up with a butt plug. She was a little intimidated, and we talked about it for a good while before we tried it. but we’ve got a few toys around, so our first time we did some graduated foreplay with toys of increasing sizes before we went at it proper. These days we still use toys, but she’ll start with the largest plug she has, and warm up time is down to 15 minutes or so.
There’s never really been “pain, strain, or discomfort” in unreasonable amounts. Lube, time and relaxation do wonders.