Listing a series of "clues", the company, Fireworld, suggested that "hacking his Facebook account" and seeing if he had visited gay websites could confirm a parent's suspicions.
The company has since taken down the article.
The post was highlighted by a French youth LGBT rights group.
L'Amicale des jeunes du Refuge's thread about Fireworld's article (in French) was retweeted by French Secretary of State for Equality Marlène Schiappa, who wrote that it showed that "homophobia and sexism have their roots in the same gender stereotypes. We will fight them together».
In its online article, since removed, the firm said that "family is fundamental. That's why the sexual orientation of your children, directly responsible for the continuation of your family, is very important to you».
The article went on to list the clues that might cause a parent to suspect that their son might be gay. The article makes no mention of female homosexuality.
They include "taking good care of himself", being more interested in reading and theatre than in football, being shy as a young boy, having certain piercings and liking female singers and divas.
It then suggested a variety of ways to be sure, including "monitoring his Facebook use", seeing "if he has visited gay forums" and "spying on his private messages».
In a response to L'Amicale des jeunes du Refuge, Fireworld wrote that "the article had the sole aim of improving search engine optimisation and was never intended to be read by humans».
"We regret not having reflected on the consequences of this type of content..." the firm emailed. "We sincerely apologise to all those who may have felt offended by this content," it added.