We've done it. I had used a hollow (big) dildo with her a few times, to her great satisfaction. I asked her more than once to describe the feeling, and one day she said, "Want to find out?"
We didn't use the big one, but a smaller one at first. I don't think I have to describe the feeling to those of you who've done it, but it sent tingles throughout my body. She said she loved seeing me so turned on. She can be quite aggressive with it sometimes, and she said she loves how I react. Her pleasure comes in pleasing me, like a good blow job.
For me, it's not a gay or bisexual thing. I could never do it in mixed company.
It's just an intimate option that we turn to every once in awhile when we're both feeling really really horny and sexy.