What he is getting at is that basic human behavior is quite similar to animals. Survive and multiply. Ultimately all our actions are advanced form of basic animal instincts. Just because we are better at thinking doesn't means we can get above these basic feelings. No matter how intelligent you are, you would still be compelled by those instincts.
And it doesn't takes much for your basic instincts to be slightly different from norm. Some can be homosexual, some might be attracted to children, even necrophiliacs. Whether or not those off tangent behaviors are acceptable depends on society. Of which homosexuality is acceptable in many, others, not so much. The idea is, two consenting adults are doing what they like with their life, let them be. We don't have any right to intrude in their private matters.