Deboche is talking nonsense, but not because it does not apply to DF. The whole point is that it is a suggestion.
The main problem seems to be that he thinks male dominated societies are "monogamous" while equal societies are "polygamous". This ignores the fact that most past societies were not monogamous, but instead held a double standard. Men often had mistresses or even other wives, but women were not allowed to be promiscuous themselves or they would be classed along with prostitutes as unsavoury women. The need for men to know that they were the father led to the demands for virgin wives and punishments for female adultery, but once they had their virgin wives many of these men went off and conceived bastards with lower class mistresses while expecting their wives to stay at home!
Polygamy of this sort benefits males far more than females. A man can impregnate hundreds of women, but a woman cannot be impregnated by hundreds of men! This leads to harem societies, as seen in many mammals, where a dominant male impregnates all the females while driving other males away. This also sometimes creates patriarchy, since the biggest males are more likely to win fights and breed, and so males become bigger than females.
Free love, or polyamory, is a very different system from the polygamy practised in many places, since the double standard is gone. This is a gender equal system, but so is true monogamy or serial monogamy as seen in some birds and dwarves. The sexist part is the double standard, where only men are allowed to be promiscuous. True monogamy, where neither partner is allowed to have any others, is sexually conservative but not innately sexist and may suit the females more for the reasons I have already given. If dwarves had inheritance, and they likely will in future, it would pass on regardless of gender.
Dwarven culture is accepting of homosexuality, but is also monogamous. The lack of sexual dimorphism also supports this, since males are usually bigger in harem societies for the reasons I have given. It does not exclude free love, but dwarves may not really want it. Whether or not anyone else's dwarves want it is irrelevant, DF is not Tolkien or any other work containing dwarves. Our dwarves can be however we like.
Small tribes have excellent population control, in the form of infanticide, frequent fighting and disease. They are environmentally sustainable only in that their populations do not reach a big enough size for them not to be; human damage to the ecosystem is largely because of the demands of a huge population.