The reason it has become political is because there are public policies directly affecting the rights of LGBTI people and their families (and, by extension, public attitudes towards LGBTI people).
I agree that sometimes people forwarding this left-wing agenda are not as respectful as they should be, overly emotional - even, and shutting down discussion. People should be able to voice their opinions, but again we should not pretend for a second that opinions are equally valuable morally or intellectually.
If the discussion were gender rights or racial marriage etc., we shouldn't even need to have the discussion in 2017. Anyone that thinks that gay marriage should not be legalised because of religious beliefs are literally wrong - intellectually wrong because the religious beliefs are not founded in truth (as anyone who has genuine intellectual curiosity, reasoning or science skills will tell you) and also morally wrong because of its emotional and social impact of LGBTI people and their families.
I appreciate that at least some here who are subtlety expressing anti-gay marriage views do so tactfully, but please unless you event a time machine do realise that your views are wrong.