To me, anal intercourse is usually the end result of alot of other anal experiences and exploration. I never had to "talk" a girlfriend into anal sex, at least not the ones I had anal intercourse with.
Most of the 20 or so women I've been intimate with enjoyed some amount of anal play, like caressing, touching, rimming, kissing, some even fingering, though most were too shy to discuss it. But their body movements and moaning and breathing communicated how they felt about it. I admit that one girlfriend hated it and yelled at me and was very mean about it, so I never touched her ass again.
I have had anal intercourse with four women in my life, all longer term relationships. All were very very open and honestly passionate about anal play, very comfortable and excited talking about it as well. With three of them, it was something that was built up to over time, exploring one another (yes, my anus was the focus too!), looking, touching, probing, playing, etc, so that eventually the next step was intercourse, which then became a regular thing.
The fourth woman was a bit different. On only our 2nd time in bed, 2nd time fully naked with each other that is, I started by caressing and sort of massaging her. She was very relaxed and as I began massaging her bottom, I spread her cheeks wide open as I did, basically because I wanted to see her anus, which I found very exciting to do. I caressed it very gently and asked if it felt good, and she quietly said "mmm yes". Later that night I was behind her ready to fuck her (vagina) and as my hard penis touched her bottom, she said, "Yes, put in in there". I was confused, "What? Where?" She whispered "In my ass, put it in my ass, fuck my ass." So that was pretty unexpected. A quiet, conservative looking school librarian who loved being fucked in the ass.
So none of those 4 women ever needed any convincing. Some women like it. Some women don't.
BTW, yes I got fucked in the ass by one of those women several times. At one point she asked me if she could fuck my ass, basically my First Time Anal. So I bought her a harness strapon dildo for her birthday. She didn't need to convince me either!