He was just joking man - I found it quite funny, although I didn't agree with the point.
Addressing your other points: do you reckon most Muslims are happy to believe both Muhammad's status as a perfect exemplar for human behaviour and his having had sex with a 9-year old? From what I've seen of you, I am not surprised to find you aren't of such mind.
How could you know that the purpose of marriage was for sex while the historical life
of the prophet shows without doubt that he wasn't interested in women, he was known
for his noble morals before Islam, he didn't have any relation with any woman and his first
woman in his life was a widow of the age of 40, he stayed with her for 25 years, she loved
him and he loved her regardless of the difference of the years between them, he was
young and she was old.
Aisha was known to be good in memorizing and a smart girl, just once she moved to the house of the prophet, she memorized the whole quran in a very short period of time , that's a good sign
that both were busy in the material world, after his death people returned to her for asking her
questions about religion.
In old age people aren't as active and alert as in their young age, the prophet shortly before
his death he asked people to gather for an important sermon and it was his final sermon before
leaving earth, he warned people to take the matter seriously and that the afterlife is real
and they'll be asked for their deeds on earth, he asked them to memorize his words to warn the
next generation about the afterlife.
Do you think he was busy in sex?