People feel different. People move differently. Are shaped differently. Maybe it's easier for her to have harder, raunchier sex with someone that she isn't in love with. I don't know as you didn't provide exactly what 'differently' is.
I think you may have touched on the issue at hand. Jealousy. Talk to her about your feelings.
You should be able to voice your concerns to your wife without it becoming an argument. If not, slam on the brakes until you can!