In his latest blog, Finns Party chair Soini wrote about participating in "demonstrations in support of marriage between men and women" in Paris.
The party leader was writing about demonstrations that took place in the French capital last Sunday, during which roughly 150,000 protesters denounced the enactment of a new law legalising same-sex marriage in France.
"As a European Union citizen I have the freedom and the right to participate," Soini told Yle.
Soini said that the original purpose of his trip to Paris was to attend the campaign launch of an ideological peer contesting a seat in upcoming European Parliament elections.
"It so happened that there was this demonstration so I joined in," Soini explained.
The conservative nationalist politician, who has also declared himself a staunch Catholic, has consistently maintained that marriage laws should not be amended to include same-sex partnerships.
"Marriage is a union between a man and a woman and that's the way it should remain. We in Finland need to understand how big this issue is. Of course someone has to dare to express this opinion," the politico added.
Soini explained that since he could not shout slogans in French, he acquired a placard for the march.
"I had my own placard provided by the organizers, with the image of a mother a father and two children," he concluded.