Its unlikely they'll recieve well a chat on chastity, so how about speaking about why marriage is so important for kids.
Every child has a mother and father. It doesn't matter how they were concieved, in a Friday night orgy, in a devoute marriage bed, in a petri dish at the IVF clinic. So, by their inherent nature, a child has a right to know their mother and their father. The best environment for a child is with their mother and father, watching and learning how the genders are different and how those differences interact with each other. Their relationship with their opposit gendered parent will determine their relationships with the opposite gender as they grow. Straight or gay, these relationship dynamics and gender role modelling are absolutely vital. To deny this is to deny reality. Its to deny nature. If homosexuals want to claim that their disorder is somehow natural, then they cannot denounce the nature of the child.
The only people who can replace a child's mother and father, is an adoptive mother and father. To allow redefinning marriage to accomodate same sex couples and allowing gay adoptions will then deny the child that right to both their mother and father. Living with butch Mum and feminine mummy is not enough. Living with butch mum and feminine mummy with "sperm donor" daddy the state over, or even as a relative [brother of one of the mums] is not acceptable.
Marriage is about protecting the right of the child to know mother and father in a family unit. Redefining marriage to accomodate same sex couples attempts to change marriage from an insitutition which protects chidlren to one that makes it adult-centric. Marriage is not about what the adult wants, its not about adult desires and wants and some process to seek fulfilment, its about providing a stable environment for child to know mother and father.
This does not deminish the marriages of individuals where children have not eventuated. All children have a mother and a father, but not all married couples have children.
Instead of redefinning marriage to accomodate same sex couples, thus watering down what society views as marriage is not helpful. What is needed is building up the current marriages and supporting those in relationships where one or both parents are afflicted with alcohol and drug issues, or abuse and domestic violence.
A redefinition of marriage to accomodate same sex couples will impact on children, on what theyr'e taught in school, they will see it as just another option, and be denined their natural rights to be protected by law and to know parents. It will impact on what Christians who seek to teach their children about God's plan. They will be conflicted, seeing society, their teachers, the media hammering them with that Jane and Jessicas "marriage" are equal to John and Sallys.
All human societies have recognised that a child does best with mum and dad in a stable relationship. Marriage has never been about the desires of the parents, while that's certainly benefiical that they gain satisficiation and fulfilment from it, marriage has not traditionally been about them. Its been about providing a stable home for kids - society has a vested interest in ensuring marriage is one man one woman. Children are raised to be good, well rounded adults, able to relate to both genders on a level that society constantly ignores. heck, in a lot of places and times, marriage wasn't even about adults choosing their spouse for themselves.
The homosexual lobby have bigger issues than the redefinintion of marriage. They have huge rates of domestic violence, huge rates of promsicuity, lower life expectancities, higher rates of cancers, particularly cervical and anal/rectal. Also drug use, alochol addiction are also wide spread problems. Plus, wer'e always reminded, alledegely higher rates of suciide amongst teens.
They shoudl be focussing their money and power elsewhere.
You need to use facts, and then bring them all back to the teaching of the Church. If you go in there going on about Jesus and the Catholic CHurch's teaching, you're going to be booed out of there.... if they're in a generous mood.