just that from a scientific standpoint (me having a degree in bio/minor in chem) I would agree with what he says. When we break down the act of heterosexual vaginal intercourse, it's not a mystery what the purpose of sex is for. This is not really debatable.
Whether or not that's the REASON why people have sex is irrelevant.

The purpose of sex is reproduction. Animals go into "heat" because that is the prime time for them to become or get another animal pregnant.

The reason why we do it is because it feels good. It brings us closer to our mates. (intimacy). It's an expression of love.

it's two seperate things and yes I am guilty of having sex for many reasons but not for the actual purpose. I am just discussing now. as a biologist this is what I believe. as a human, I love sex just as much as the next person. It's fun.