Because next time he may be the she. Snails are hermaphroditic and can change sex at will or be both sexes at the same time. Group sex is not uncommon. And snails are also able to mate with themselves. Then let's look at the chimpanzee, so much like us that one can't tell the difference between our blood hemoglobin and theirs.
Chimpanzees are highly social animals, actually promiscuous.
While among humans, the family that prays together stays together, among chimps a more earthy experience strengthens the ties that bind. Even cross generational sex is engaged in with infant males copulating with mature females and mature males having sex with adolescent females. Females often take all comers, whether they are experiencing estrus or not. Sex is definitely NOT done solely for procreation. So it seems that if we learn anything about human sex roles in nature it is that nature loves variation.