Thirded. Dan's just being nasty. Pressuring a man to get a partner, or marry and produce babies, is no more okay than pressuring a woman. The pressurers will never get that it's not okay, and it's part of the same larger culture that hates queers, even if the writer's in little to no danger of being disowned or murdered. Next time though write to Miss Manners, she'll have an actual pithy comeback, not a "you whiny little shit" style lecture.
Letting poly people who think polyamory is a relationship orientation (I'm agnostic on the question) have their say in his column was bs, no surprise.....just posting one woman's tweet calling him a bigot and making no actual argument (there were some thought provoking ones over in The Stranger's comments). But no, he hasn't pointed out as a fact that all people are nonmonogamous, he has stated that opinion as if it were a fact. It's no truer than saying everybody's bisexual, or even sexual.
His complaint is minimal, but "coward" is certainly harsh.