I think it's because polygamous relationships lend themselves to manipulative and abusive behavior more easily than monogamous relationships do. Over time that increase in probability of negative outcomes turns into social change and prescription against it.
But the problem there is still the manipulative and abusive behavior, not the polygamy. It's absurd to argue against healthy polygamous relationships just because there are unhealthy ones.
That's not what your source says, it says that the heterosexual:homosexual ratio is 11:1 among those it found to be pedophiles, which is lower than the 20:1 ratio in the general population, implying a higher (but less than 2×, not about 10×) likelihood of pedophiles among homosexuals than heterosexuals.