It's part of their agenda, which is religous instead of financial. Similar to pro and anti abortionist fighting for any abortion related bill in any state. Or the gun lobby fighting any bill in any state that would regulate or restrict anything related to guns.
Serperation of church (or any idealology that include morals) and state in the USA isn't 100%. Originally, the first amendment only referred to the federal level. States were allowed to have their own religously based laws, as long as these laws didn't conflict with federal laws. Over time, this evolved to the point that only remnants of the old laws exist. Note that adultery is still illegal althogh rarely prosecuted, in some states:
http://en.wikipedia.org/wiki/Adultery#Criminal_penalties
Regarding age of consent, a few female teachers in certain states received jail sentences for having sex with 17 year old boys, something that wouldn't be a crime in states where the age of consent is 17 or 16. These cases couldn't be appealed because the USA specifically allows the states to determine most of their laws, including morality based laws.
My previous comments about pedophilia were as another example of sexual orientation, clearly much more "unnatural" than homosexuality, and causing harm if the desires are acted upon. Pedofiles experience a sexual preference for children (in the USA this term is expanded by law enforcement to include any minors, apparently to stigmitize sex with minors (up to age 17) further still). Curently there is no cure: Although pedophilia has no cure at this time, various treatments are available that are aimed at reducing or preventing the expression of pedophilic behavior, again from a wiki article:
http://en.wikipedia.org/wiki/Pedofile
Heterosexuality, bisexuality, homosexuality, and pedophilia are all sexual orientations. Currently, most societies consider heterosexuality as normal, and pedophila as very abnormal, with bisexuality and homosexuality somewhere in between. The USA is not a libeterian country, so it's not a truly "free" society. This means that the government can choose to discourage or simply to not encourage certain behaviors, even if it's not clear that any harm is being done. Regardless if you think it's right or wrong, it's the current state of the USA and most governments.