Insecure guys need validation.. Honestly that being said he is doing it to get off and not validation. We can jerk off 2 anything. Your girl obviously isn't satisfying him in bed. Blame it on your friend on not him.. Girls don't even know how to ride the D today but think that laying there like a dead fish is ok. This dude has every right to jerk his dink to anything he pleases.
... because sex...
To summarize it all in one word: Curiosity Seeing something new, or an experience with someone new, is thrilling, because it's out of curiosity.