Non. When we acknowledge which movements were in firm opposition to the legalization of gay marriage in France, the dominant group was and continues to be French Catholics. And from the political spectrum, political parties representative of "La Droite". Even as the prediction from the Pew Center,
http://www.pewforum.org/2011/01/27/future-of-the-global-muslim-population-regional-europe/ points to an increase of French Muslims to 10.3% by 2030, I am not sure how anyone would anticipate that such low percentage of French Muslims would somehow succeed in reversing GLBT Rights in France. If any potential for a reversal of the 2013 Legislation legalizing Gay Marriage and Right to Adoption, it would be because Right Wing Political Parties succeed in winning a majority of seats in the General Assembly and the Senate while the Executive Branch is dominated by the Right Wing too.
Further, it is to be noted that such RW parties have managed to alienate voters representative of our population of immigrant origin, meaning from Northern Africa and Sub Sahara Africa also representative of French Muslims.
In the big picture, Muslim youth in France is far more concerned about the socio economical stagnation affecting their group (especially youth living in our Projects) than Gay Marriage. Far more concerned about their facing the highest unemployment rate than Gay Marriage.
If it is the case that allowing immigration from extremely conservative cultures that have high numbers of children results in much higher risk of regression in liberal policies and, as a result, an increase in the risk of abuse of human rights in the future (women, LGBT, regressive criminal justice, etc.), is this risk and the harm that results from it acceptable? Is the fear even legitimate?
The fear is not legitimate. Such fear has been fueled by fear-mongerers who envision a European Community dominated by the Shariah Law, predicting loom and doom.
Ironically and speaking of women in France, we saw the onset of the State interfering with the rights of religious persons, namely French Muslim women via the ban of the wearing of the veil in public spaces. Meaning that our secular State trampled over the rights of women in my country of origin.
"regressive criminal justice" : again one would have to envision the French Republic whose State is secured as a Secular State under the 1905 Secular Law adopting the Shariah law as its criminal and civil codes governing our justice system. Further, modifications of the French Constitution which can only happen if and when approved by the Conseil d'Etat or Conseil Constitutionel. That might make for a cheap political fiction paper back book where somehow Muslim clergy members would not only control the Executive and Legislative branches(while somehow 10.3 % of Muslims would present a majority of voters electing a majority of Muslim legislators seats in our Parliament) but also the body who guards our Constitution.
What would you tell a gay couple living in Belgium who fear that their rights are at high risk of being abused within 20 years due to the substantially increased concentration of Muslims in their country that will occur over that time?
I would reconnect them to the reality of the content of the Belgian Constitution and make sure they get a 101 on Civic Instruction so they may set aside their fear of "their rights" granted in 2003 being "at high risk of being abused within 20 years" due to a "substantially increased concentration of Muslims in their country".
I would equally proceed with a French Gay couple fearing their rights "are at high risk of being abused etc etc....".
What if education and current liberal policies supported are not enough to reverse this trend?
What trend? The study as well as your comments have been speculating on a so called trend. While totally dismissing how fictive the scenario of a political take over by Belgian Muslims is. As fictive as it is in my country of origin.
Must supporters of liberalism in these countries in Europe wave the inevitable white flag as societal attitude shift towards social conservatism takes hold?
And a social conservatism the cause of which you somehow attribute to an increase in Muslim immigration/population in "these countries in Europe"? Are you under the impression that "La Droite" in France representative of social conservatism is somehow influenced by the diatribes of conservative Muslim clergy members???? Are you oblivious to the xenophobic traits of such RW political parties and their electorate? Their opposition of immigration policies promoted by "La Gauche" (The Left) who are representative of social liberals? Soon to come a claim that Le Front National led by Marine Le pen is allied with conservative Muslims?
The reality being that French Muslim voters are far more inclined to support liberal parties because those parties from "la Gauche" are not affected by the need to define what a "True French" is while exhibiting nationalist stances and are not affected by xenophobia. Whereas parties representative of conservatism hold anti immigration stances which can only affect our population of immigrant ancestry and generations to come.
To be clear : the increasing popularity of the FN in France is due to the dissatisfaction with Holland's (Socialist Party) economical platform not because the French are under the influence of a Muslim population, who by increasing to 10.3 % by 2030 would somehow cause a shift towards social conservatism. The problem is that those disgruntled voters, due to the Holland's administration failures to stabilize the French economy, sacrifice their social liberalism at the altar of the appearance of stability and hopes promoted by the Right Wing parties. In no way is it to be confused for meaning that those shifting to the Right voters are embracing the FN social conservatism.
Women's Reproductive Rights, Gay Rights, and our Justice system governed by the Code Civil and the Code Criminal de Justice are not going to be changing for the worst based on any trend for French voters to be hesitant to support candidates from La Gauche motivated by their profound dissatisfaction with the PS failure to boost the French economy by reducing unemployment rates etc... They remain a majority of social liberals who find themselves having lost faith in the promises of economical stability formulated by the PS dominating the current administration.