* Four Ukrainian women stage topless protest as pope speaks
* Vatican paper reacts to court ruling on lesbian parents
* Gay marriage opponents march in Paris (Adds protest by topless activists)
By Catherine Hornby
VATICAN CITY, Jan 13 (Reuters) - The Vatican underlined its opposition to gay adoption on Sunday as same-sex marriage supporters staged a topless protest in front of the pope in St. Peter's Square.
The Vatican paper L'Osservatore Romano published a response to an Italian court's rejection of an appeal by a father who feared his son would not have a balanced upbringing if he lived with his mother and her female partner.
The Court of Cassation ruled it was "mere prejudice" to assume that living with a homosexual couple could be detrimental for a child's development.
While gay rights group Arcigay called it a "historic ruling" for Italy, where it is illegal for gay couples to adopt, Catholic leaders were quick to defend the traditional family unit.
L'Osservatore Romano, the 151-year-old mouthpiece of the Holy See, ran an editorial that sought to play down the court ruling, saying that children often grow up in difficult circumstances without a mother or father.
"But no one believes that these situations should be created just because in some cases they don't cause damage," wrote Adriano Pessina, director of bioethics at the Catholic University of the Sacred Heart.
"The human is the masculine and the feminine ... the monogamous family is the ideal place to learn the meaning of human relations and is the environment where the best form of growth is possible," he wrote.
He reaffirmed the Vatican's view that no one has the "right" to children that he said gay couples who want to adopt are claiming.
In Paris, hundreds of thousands of people protested against President Francois Hollande's planned legalisation of same-sex marriage.
The Vatican has become increasingly vocal against homosexual marriage in recent months. Pope Benedict strongly reaffirmed the Church's opposition to it in December, saying heterosexual marriage had an indispensable role in society.
While the pope was giving his weekly address on Sunday, four women from the Ukrainian Femen group who were in the crowd, pulled off their T-shirts to reveal the slogan "In Gay we Trust" painted over their bodies.
Screaming "Homophobes shut up" as the pope started his Angelus blessing, they provoked angry reactions from pilgrims in St. Peter's Square. One woman in the crowd started hitting the activists with an umbrella, calling them "diabolical".
Italian police grabbed the protesters and pulled them away from the crowd. (Additional reporting by Carmelo Camilli; Editing by Robin Pomeroy)