In fact sex between the 'same' is the most primitive kind of sex practised by living organisms. That shows well beyond doubt that the basic purpose of sex is not reproduction and the pleasure or bonding part is more essentially ingrained for the same sex. It does seem that after sexual dimorphism occurred, nature chose to ride piggy back upon sex between male and female to achieve procreation. But as can be seen from the wild life, it still did not result in heterosexuality.

There are some examples from higher animals too who don't reproduce sexually, but they still have sex with the same-sex.
SEXUAL NEED FOR THE SAME-SEX IS THE BASIC PURPOSE/ DRIVE OF SEX, EVEN BEFORE REPRODUCTION STARTED HAPPENING SEXUALLY