I get asked this every so often and in the moment I don't really have an answer other than "it just really turns me on".

I suppose I should look at it from the perspective of what turns me on.

I love that slightly jealous feeling. I love it when the guy is bigger than me both in height and size. And also if he is better looking too. I enjoy the feeling like she is being naughty (and cheating on me) too. I get turned on especially if he doesn't know that she is my girlfriend. Either he thinks that she is cheating or that she just doesn't have a bf. I get turned on when I can hear/see that she is getting alot of pleasure.

There are probably many more but I can't think of them right now.

I suppose part of the reason is that I love her alot and I know that I am not particularly big or good in bed and I just want her to have the best.

But for the rest I am at a loss