Homosexuals make up 5 – 10% of the population; if we assume that homosexual marriages will have the same divorce rate as heterosexual marriages (may be unjustified) and that all homosexuals will get married (may again be unjustified), we have a rough estimate of 2.5 – 5% increase in the divorce rate. This is almost insignificant from a statistical standpoint and will not effect the heterosexual divorce rate, which is statistically significant! Since homosexuals cannot presently marry (in most places) the current rates cannot be attributed to them. Additionally, your argument seems to be of the slippery slope variety that if homosexuals can legally marry society will instantly devolve faster than you believe it is now; however, there doesn’t seem to be any data to support this extension.
In any case, where did you get the data indicating only 10% of homosexuals are monogamous for a just a few years?
Doesn’t the equal protection clause in the constitution guarantee that homosexuals should have the same status and rights as heterosexuals?