Obviously, one of the duties of a conscientious Catholic parents is to successfully pass on to their children belief in Catholic moral teaching. We believe, after all, that living according to the Church's moral teachings is key to both the happiness and salvation of our children, and both of these are things we ought to care about a good bit.
This much, at least, is widely agreed upon. Why, however, should that be a reason not to want your children exposed to the children of a same-sex-couple? Isn't that simply a great chance to talk about the Church's teachings about marriage and sexual morality?
Frankly, I (and I think many other Catholic parents) would rather not have to rush that one. Why?
Both thinking back to my own childhood and also about my children (currently ages 8 through 1.5) one of the things that stands out to me very clearly is that children are naturally dualistic. There's a reason why the fairy tale is a genre so enjoyed by children -- children like clear heroes and villains. The adult my be interested in why it is that the wicked witch became wicked, and whether she really thought she was wicked, but to a child, the fact that she is wicked is all they need. Heroes do good things, villains to bad things, and children under the age of 10-12 have a great deal of difficulty seeing people in between.
This is one of the reasons why my wife and I are very careful about what books and movies we expose our children to: Once someone is "the good guy", everything he does is admired and imitated. The flawed hero is not something that children are good at understanding. You see this when children interact with their real life friends as well. The girl down the street who is a "best friend" one day is "that mean girl I just hate, hate, hate" when she offends.
Thus, when I seek to keep my children from running into certain types of sins (divorce and remarriage, adultery, fornication, homosexual relationships) it's not so much because I don't want to explain these sins to my children, though that's part of it. It's more because I'd rather not have to deal with the delicate balancing act of trying to explain the "hate the sin, love the sinner" concept to a mind which is little capable of making the distinction.
"Daddy, there's a new girl in RE class named Heather. Instead of a mommy and a daddy, she has two mommies. How does she have two mommies?"
"Well, Virginia, only a man and a woman can make a baby together, that's how God made us. Miss Jennifer and Miss Jean may have adopted Heather, or maybe one of them had Heather before they met each other."
"But are Miss Jennifer and Miss Jean married?"
"No, women can't be married to each other. It would be very wrong for two women to live together as if they were married. God tells us that only men and women can marry because only men and women can have babies together. But some women try to live together as if they were married anyway."
* * * *
"Daddy, I told Heather that her mommies are not really married and she cried. Then she said I was a big liar. And one of the boys asked her if she was a dipe. What's a dipe?"
"I think the boy was trying to say a very mean word, and I don't think that you should use that word, Virginia. It was very mean of the boy to say that to her."
"But why did she say I was a liar? Her mommies aren't married. You said so. They can't be."
"Sometimes people don't like to hear things even if they are true, honey. Maybe it's better if you don't talk to Heather about her mommies."
* * * *
"Daddy, I asked one of Heather's mommies, Miss Jennifer, if she was really married, and she said she was! Then I told her you said God didn't like that and she said you must be judge-mental. Are you judge-mental, Daddy?"
"Not everyone understands what God tells us about marriage, Virginia. When Miss Jennifer said I was judgmental, she meant that she disagreed with what God tells us about marriage."
"Miss Jennifer said that God made some women so that they love each other, and so God means them to get married. Is that true, Daddy?"
"No, dear. Miss Jennifer is wrong."
Yes, it could be done, but no sane parent wants to get into these situations.
It's not a teaching opportunity, teaching only works well with people able to understand. It's an aggravation and confusion opportunity. Children have three modes when dealing with these situations: Assuming something is okay because the person in question seems nice; deciding to loudly despise the person because "he's bad"; and pestering all people involved with awkward questions. Since none of these are desirable, parents would prefer not to have to deal with the "two mommies" kind of problems unless family connection forces them to. Just as they'd rather not delve into the fact that the nice woman named Phyllis who comes to family functions with Uncle Edgar is not actually his wife, and will fail to draw a "teaching moment" from the fact that Aunt Belinda's oldest child was born two months after she got married.
Of course, family connections often result in children being forcibly exposed to sex out of wedlock, divorce, adultery, etc. But at least in my own experience, when these realities do in fact make themselves known to children the results are usually less than illuminating. Children are inveterate side takers, and if they do not (as they are surprisingly able to do) remain blithely unaware of a situation going on before their very eyes, they will tend to be the ones who say hurtful things loudly at gatherings which leave all the adults glaring at each other.
And if it's difficult to explain to children about a nasty divorce without the children deciding they need to make their moral indignation known by behaving badly in public toward one of the parties, it is that much harder to explain a situation to a child in which the sinners are apparently happy and united in their sin. If one makes a big deal of it, the child is likely to take things to far and attempt to do a little of their own evangelizing (with disastrous results.) If one is circumspect, the child will assume this is just fine, and is unlikely to believe you years later when you attempt to explain that such things are wrong.
While it may seem like singling out homosexuals for special scorn, the "same sex marriage" is perhaps the most difficult lifestyle sin to explain to children. Divorce, because it fractures a family, is naturally disliked by children. Adultery, if it somehow becomes known, will be so only in its home-breaking sense, and thus rejected similarly to divorce. Same sex marriage, however, is unique in claiming to be a marriage when it is not. And thus is by far the most difficult to explain to children. I don't think it's unjustified for parents, who care strongly about presenting a good example of what marriage really is to their children, to not want to have such an issue brought up to their children before the children are of sufficient mental and moral maturity to be able to understand the situation and the Church's response to it.