This argument is a bunch of horseshit for a number of reasons, the main being that the numbers simply do not add up. Europe has already one of the highest population density in the world, so positive population growth is the last thing they would want. Consider this:
Netherlands - 387 people per square kilometer Belgium - 337/sq km UK - 244/sq km Germany - 233/sq km Italy - 192 sq/km France 109 sq/km.
By comparison: US - 29 (that is twenty-nine!) people per square kilometer
Japan - 336/sq km India - 318 /sq km People's R. of China - 134/ sq km
Second, negative population growth, mainly in some Eastern European countries, are due to outmigration and high mortality rates brought about "democratization" and "privatization" (read: looting) of the public sector. The folks who are concerned about that, mainly the nationalists and Catholics who make Bush look like a wussy liberal, are indeed using it as their reason to ban abortion and promote multi-child families, but their opposition to homosexuality is mainly on moral grounds.
I think that opposition to homosexuality, here and elsewhere, is not functional (i.e. to achieve some policy goal or utility, such as maximizing reproduction) or religious (as non-white non-xtian populations can be even more homophobic than xtian whites) - but cognitive.
To make a long story short, our cognition operates by lumping and splitting (Eviatar Zerubavel, The Fine Line, NY: Free Press, 1991). Individual things are seen as elements of categories. Things that are a part of the same category are "lumped" i.e. seen as more similar to other things in the same category, and "split" from things that belong to different categories. The perceived similarities are cognitive rather than physical i.e. the differences are in the eye of the beholder rather than in the physically measured qualities.
To illustrate, consider a crowded pub or a restaurant. If you placed a microphone somewhere in the middle and listen to what that microphone picks up through a remote speaker, all what you would hear is indistinguishable cacophony of voices. Yet, people who are in that pub or a restaurant have meaningful conversations with one another - in fact there are hundred conversations going on at the same time in that pub. Somehow, the participants of those conversation "tune in" to the voices of their interlocutors while "filtering out" all other voices that belong to other conversations. Stated differently, they "lump" the utterances of members of their party in the whole "our conversation" and split it apart from utterances that belong to "other conversations."
Now, if everyone in that restaurant does the same thing, i.e. tunes in to "his/her conversation" and filters out what people at the adjacent tables are saying - everything goes smoothly. But if somebody violates that rule and starts mixing up things that belong to different conversations - people will get really pissed off. Just imagine: a bunch of lbo-talkers have a nice conversation about general suckiness of capitalism, but then one of them interjects "Do you know what those folks at the table behind me just said? Jennifer Lopez has an affair with Michael Jackson!" Or the same person turns around to the table where folks are having a conversation about celebrities and says "You know what, folks? All that celebrity crap is a capitalist plot to keep people in line."
The same pertains to homosexuality - many people perceive it as lumping items that are conventional split apart. And that makes some of them very apprehensive.
People react negatively in such cases, because such transgression of conventional boundaries destroys order (no matter how arbitrary) and creates chaos. People are generally aversive to chaos, especially when they expect order. They may enjoy a little bit of chaos every now and then, but only in controlled situation (e.g. at a rock concert). But they are generally pissed if things do not work as expected. If I am on plane heading for, say, Des Moines, and the pilot announces "Guess what, folks. We are going to New York City instead." I would be pissed not because I like Des Moines better than NYC, but because the airline's breach of my expectations threw chaos into my life.
Of course, the way people handle uncertainty and the unexpected depends on their cognitive disposition. Those who are cognitively rigid would be more horrified by breaching conventional expectations than those who are cognitively flexible. Some authors talk about boundary-orientation vs. center-orientation. Those who form cognitive clusters mainly by drawing boundaries around them (i.e. separating border items from those that belong to different clusters) tend to be more rigid about violating those boundaries than the folks who form cognitive clusters by defining its central characteristics (i.e. everything similar to it belong to the cluster) and are less concerned whether borderline items do or do not belong to that cluster.
The corollary of that argument is that ideological differences among people tend to be not functional (i.e. based on preferences linked to some utility or broadly understood material interests) but cognitive. The cognitively rigid, boundary-oriented people (bunker-heads and conservatives, as some would say) tend to be attracted to authoritarian ideologies (or more accurately - weltanschauungen) regardless of their political orientation (left or right), religion or cultural background because such ideologies better fit their mode of their cognition by emphasizing adherence to conventional norms and discouraging transgressions and uncertainties (with which these folks cannot deal very well).
By contrast, people who are cognitively flexible (flaky and undecisive, as some would say) and center-oriented in constructing cognitive categories tend to be attracted to liberal ideologies - again regardless of their political, religious, or cultural background - because these ideologies (weltanschauungen) are a better fit for their mode of cognition by emphasizing relativity of norms and expectations and flexibility in arranging things into different patterns.
Wojtek