"Problem when making love" joke

A couple went for their yearly checkups.
The doctor saw the man first and asked him how he'd
been feeling. " I have one problem, Doc," he answered.
"The first time my wife and I made love everything
was just great, but the second time I sweat a lot"
The doctor completed the physical and then examined the wife.
"Your husband says that the first time you two
made love, that things were great, but that
he perspired the second time". Do you have any idea why this might be
asked the Doc.
" Of course I do!" she sxclaimed.
" The first time was in winter and the second time
was in summer".