Boys and girls ending up in same pair

Question

100 boys and 100 girls are randomly assigned into 100 pairs. What are some ways to calculate how many groups get 2 girls?

I can calculate this using a computer program in Python, that I know, how do I think about it mathematically, that I want to learn.

# cook your code here
import random
list=[]
girls=[]
boys=[]
duplicate = 0

for i in range(200):

          r=random.randint(1,100)
          if list.count(r) < 2: 
              list.append(r)
              if i % 2 == 0:
                  girls.append(r)
                  if girls.count(r) == 2: 
                      duplicate += 1
              else:
                  boys.append(r)

          else:
              i -= 1


print(duplicate)