Because of what you read in the the writers of imperial Rome. Some people are indeed homoerotic by nature. But others, as Aristotle noted, develop this as an acquired passion. Homoeroticism is, to a large degree, socially constructed. It turns out that where homoeroticism is granted full social sanction, as it was in Rome, it flourishes -- so much so, that one writer noted that the emperor Claudius exhibited an unusual trait: he was sexually interested in women alone!
The social history behind this piece is clear: once they've experienced sex with other men, Catullus tells us, men are unsatisfied with what their new wives provide them. Notice that the poet is unconcerned about the husband's dallying with other women -- it's the other men around that threaten the marital union.
The losers from all this will be the vast majority of women. With full social sanction given to homoerotic activity, the historical precedent suggests that tomorrow's women will have a harder time finding and holding on to suitable men. As women will suffer, so will the vitality and stability of the nuclear family.
Basically, it is the orientation equivalent of "once you go black, you never go back." (Once you try man, you're always a fan?).
These arguments always amuse me, because they seem of the sort that would only be persuasive to folks hard at work suppressing their own queer tendencies. Speaking as someone who would probably suffer few immediate social consequences to coming out as gay or bi, much less engaging in a little "experimentation", I can honestly say I've never really felt the urge to hook up with a fellow possessor of the Y chromosome. Go figure.
Klinghoffer says that "if you want to disagree with this analysis, you'll have to explain why the historical parallel doesn't apply." Okay, sure. If we're accepting that homoeroticism is socially constructed, then we have to accept the same thing to be true of heteroeroticism. It should not surprise us that in misogynist societies where a) women are constantly devalued as inferior and subordinate beings and b) same-sex relationships were a viable alternative, that male/male pairings would be seen as superior and normatively preferable. In other words, I Blame The Patriarchy. The way to keep gay marriage from being a threat to women, unsurprisingly, is by breaking down the mentalities that say women are inferior creatures (the same tactic, conveniently enough, for dispatching many other threats to female equality. Fancy that!). Where women are seen as equal, then I have full faith in their ability to compete in the market of relationships.
* Berman says that lesbianism did not increase, and writes "I leave it the reader's basic grasp of anatomy to figure out why in ancient Rome a man who found pleasure in a woman, could also find pleasure in a man, while the record shows that a heterosexual woman rarely found sexual satisfaction in the company of another woman." Well then I say, thank God for technology! And, you know, non-penetrative sex.