I definitely used to feel this way about watching my GF with new guys but then eventually that excitement starts to wear off a bit. Then I think it transitioned more towards getting excited over the circumstance. Not just a "new" guy fucking my GF but "who" is fucking her or "where" are they fucking her.
This is where playing the bar game, going to an adult theater, etc. adds a little bit of excitement to the experience. I think we find it more exciting when it's unplanned, spur of the moment, at an unexpected time or in an unexpected place.