The mans g-spot is located about 4-6 inches inside of our rectum. I'm sure he was just trying to reach the peak of pleasure.

I wouldn't worry about it though. We all masturabte, and if you say you don't, you lie. If it goes a step further though to wanting an actual man to do the job, then I'd be concerned, on a cheating level.

He could be bi or bi-curious for all you know, but as long as he has sex with you and you only, then who cares.