We're both bi and play with bi people so we are both very oral, but we also both enjoy intercourse. Watching, being watched and doing it. We've been in this situation too, but in the same way as biblonde. I (wife) find it very frustrating.
When a guy goes soft, it makes you second guess yourself as to why, and if he cums in just a few strokes it just leaves you feeling frustrated. That's not to say that I don't have fun: Defininetly do, but I guess I'm interested too on why this is such a problem?
Fortunately for me, hubby can and does get hard repeatedly and lasts for a long time so I always get enough sex, just not always exactly when I want it. I don't want to make him stop having intercourse just because the other male finished early, or couldn't perform. Usually I just continue to play with her and my hubby (benefit of our both being bi, voyeuristic and exhibitionists all in one package! LOL!).
Do have to say though that this is one of the main reasons I prefer group sex. Love watching people have sex, but also love having sex. With a group: I get both without caring whether or not the other guy can do it. Plus the additional benefit of being watched by several while having sex.