I am doing a quick analysis here

There are approximately 17,000 planes with 1,00,000 daily flights and there are approximately 1,30,000 male pilots and 3,20,000 flight attendants.

Avg number of pilots per flight = 130000/100000 = 1.3. Avg number of flight attendants per flight = 3,20,000/100000= 3.2.

Average person has sex 127 times per year .

So per day it will be 127/365 = 0.3479.

There are 2086697783 males between 21 to 65 in the world(Minimum age of commercial pilot is 21 and let's consider maximum age is 65) .

There are 2050835628 females between 18 to 60(Minimum age of flight attendant is 21 and let's consider maximum age is 60).

The probability that the woman having sex with pilot is

=Number of male pilots/number of males between 21 to 65 in the world

= 1,30,000/2086697783

= 0.00006229939.

The probability that the man having sex with flight attendant is

= Number of flight attendants/number of females between 18 to 60 in the world =3,20,000/2050835628

= 0.00015603395.

The probability that the flight attendant will have sex with a pilot is (P(f))

= Avg number of flight attendants per flight*No of times Average person has sex per day*probability that the woman having sex with pilot

=3.2*0.3479*0.00006229939

= 0.00006935666.

The probability that the pilot will have sex with a flight attendant is (P(p))

= Avg number of male pilots per flight*No of times Average person has sex per day*probability that the man having sex with flight attendent

=1.3*0.3479*0.00015603395

= 0.00007056947.

The probability that the pilot and flight attendant who are having sex are on the same flight is

P(f n p)=P(f)*P(p)

=0.00006935666*0.00007056947

= 48.9446273717*10^(-10).

So the odds of a flight attendant having sex with a pilot in any given day is approximately 48.9446273717*10^(-10).

So according to probability this type of incident occurs every 559816 years

BUT it is not very unlikely

for example

The odds of a person hit by a lightning in any given day is approximately 39.1*10^(-10).

The odds of a person dying in a plane crash is 90.0*10^(-9)

LAST and LEAST the probability of this earth existing is 10^(-10^(123))

P.S:There should be many factors taken into consideration,but this is the quickest way i can come with