(Beis Shamai): She needs two Edim for every Bi'ah, or she has Bi'ah in front of a lamp.

Question: Was it not taught that one who has Bi'ah in front of a lamp, this is despised?!

Answer: Our Mishnah means that she has Bi'ah, and checks [the Edim] with the like of a lamp.

(R. Ze'ira citing R. Chiya bar Ashi citing Rav): If she had Bi'ah with an Ed [and it was lost], she is forbidden to her house (husband).

Question (R. Ze'ira): If when the proof (the Ed) is intact, she is permitted (even before she checks it, even though she could check it. All agree that she may have Bi'ah the entire night without a lamp), when the proof is not intact, all the more so [she should be permitted]!

Answer (R. Yosi): There (when it is intact) you can clarify (check it later, and Chachamim's decree to have Bi'ah with Edim is fulfilled. When it was lost, the decree cannot be fulfilled.)