Prior to the general use of air conditioning in the summer the temperature in the homes was the same temperature as outside, maybe hotter. Houses were built with the ideal to create air flow. One of the common house types in Delmar is the American Four Square which has two windows located in each corner of the house (in turn there is one room in each corner of the house) to help with air flow. Altho many houses had sleeping porches for air flow the high temperatures should indicate low sex activity for our great great grandparents.
Altho Spring is the time of year when love is in the air in the 1800s and early 1900's this was a farming society and everyone did physical work, so simple exhaustion from farming and farm house labor would also create low sex activity. To see if summer and sex activity correlated I looked at the 1900 census for the Village Of Delmar, Delaware for birth dates. At that time there was a population of about 427 and 104 homes.
number with Jan birthday month 23, Conception in May
number with Feb birthday month 49, Conception in June
number with Mar birthday month 46, Conception in July
number with Apr birthday month 22, Conception in August
number with May birthday month 42, Conception in September
number with Jun birthday month 38, Conception in October
number with Jul birthday month 24, Conception in November
number with Aug birthday month 42, Conception in December
number with Sep birthday month 30, Conception in January
number with Oct birthday month 38, Conception in February
number with Nov birthday month 41, Conception in March
number with Dec birthday month 32, Conception in April
So lets have a round of applause for our great great grandparents only in August do we see a decrease in conception for April children with the actual peak conception times being in June and July. This is of course is an unscientific method that just looked at one variable the birth month. Items not taken into consideration would be weather and temperature in birth year, the geographic location of birth (probably a high percentage were not born in our area) and any kind of accurate gestation period.
See R. Clyde White "The Human Pairing Season in America"
and Birth Month and Maturation Rates