1. Marriage is something christians believe to be strictly between a man and a woman, thats it. its not some hate filled belief, two guys getting married is not marriage in their eyes. its not homophobic.
Now, there are a lot of christians who do have some level of hate towards gays, are homophobic, discriminatory, etc.. but if they are, they arent a TRUE christian. thats not living "like christ"
2. Its pretty unclear if homosexuality is actually considered a sin, the only things in the bible that would point to that comes from the words of people, not the word of God, and remember, the bible was written by people, well before there was any kind of knowledge or acceptance of homosexuality that there is today. What IS a sin, is to judge, hate, condemn, other people. And all sins are equal to God, So a christian judging a homosexual is just as sinful as a homosexual might be (if it is a sin). and everyone sins, every day, the most perfect christians sin, they are incapable of not sinning, a homosexual could be just as "sinful" as the most perfect christian.
A true christian would have nothing but love and grace towards a homosexual. Thats not homophobia.
anyway, back to the topic of if homosexuality is a sin, like i said before, the arguments ive heard before about it being a sin come from the words of PEOPLE, i.e. sinniers themselves. and its more along the lines of "sinful sexual behavior" between the same gender, not same gender love.
If i remember right the only thing coming from the word of God that could be associated with homosexuality, or heterosexuality/sexuality in general, is more along the lines of if its exploitative, manipulative, dishonest, selfish, destructive, etc.. its sinful. if the motives are pure and honest, if its a faithful love, there's nothing I know of from Gods word that would say its a sin. And regardless, only God can judge the sins of others, not us.
So sparknotes: a true christian would be the opposite of a homophobe, he would only show love and grace towards gays.
And this is all coming from a non religious, pretty much atheist. I just have studied a lot of christianity because i find it interesting, grew up going to church, and find that regardless of your faith or lack of, striving to be like christ is about the most perfect way you can live your life.