I prefer to be second, not because I like the other guy's semen, but because I like being excited while he's fucking her, whether it's my wife or some other woman. The excitement of watching my wife having sex with another man is way more intense before I've blown my load than afterwards. When they've finished, my wife is relaxing from an orgasm or two, is full of cum, and still wants me, makes me unbelievably hard. Going in and wallowing around in the slop is further arousing because all I can think of is how sexual my wife is.
If I go first, after orgasming watching my wife having intercourse is interesting, but more of an intellectual event. I'm calm and temporarily depleted.