King David was a big stud in his old age

Today's daf (Sanhedrin 22) relates the following story:

(Gemara - R. Yakov): Avishag (the girl picked for David) was permitted to Shlomo, for a king may use what another king used;

1.She was forbidden to Adoniyahu, for he was not a king.

(d)Question: What was her relationship with David?

(e)Answer: "Va'Tehi la'Melech Sochenes va'Tesharesehu."

1.She asked David to marry her; he told her that he may not take a 19th wife.

2.Avishag: That is a lame excuse! (Really, you are too old and weak to have intercourse.)

3."Va'Tavo Bas Sheva El ha'Melech ha'Chedrah" - David called Bas Sheva, and she cleaned herself 13 times (Rashi - they had intercourse 13 times and after each time Bat Sheva cleaned out the semen) in front of Avishag.

According to the Gemara King David, who was a weak old man at the time in his 60s was able to have intercourse 13 times in a very short period. Quite unbelievable.