Is it normal to lose erection after 20 minutes of foreplay?

After multiple sexual encounters with several people, I have noticed that after 15 to 20 minutes of foreplay, I lose my hard on. I am 23 and fuck my fiance regularly (or jerk off to porn), but when I have sex with others I have a hard time saying hard if we don't actually fuck. I think it's a mix of being unfamiliar with other people and most importantly, the desire to penetrate someone after I'm hard. Is it normal to lose the mood for a guy if there is too much foreplay? (We are obviously in an open relationship; that's not what is in question)