I am from an evangelical originally Anglican) background and spent my time with lots of evangelical and sometimes with liberals too! 
I know the Bible has various passages which appear to say that homosexual sex is wrong and I know that some Christians would say this means all homosexual sex is wrong. I think this is based on how this passages are translated these days and how they are interpreted.
In the past some of these passages may have translated a little differently and their meaning can be a bit obscure. The reason that some Christians will feel these passages refer to all homosexual acts/relationships is (I think) because they are taught to interpret them in this way. This is how they have been interpreted for a while (could someone wiser be able to say more!).
A number of Christians would argue that these passages are talking about different things, for example Leviticus referring to homosexual acts as a form of temple prostitution, which was rife in the time that these books were written. The book of Leviticus was talking about a different situation from gay men and women now, it was talking about temple prostitutes. There is more to it than that and I wonder if anyone can say more?
There have always been liberal minded Christians who were very happy with gay relationships but now a growing number of evangelical Christians are also trying to understand what the Bible says about homosexual/gay relationships (and the answer seems to be not a lot!) but what it does say needs to be properly interpreted.
I know some might read this who are not the least bit interested in what the Bible says so it will mean nothing to them, and that is fine, but to those of us who have made a decision to take the Bible seriously, though not always literally, and to take it in context it is very important to understand what it really means. Here are a couple of links I found.
www.upworthy.com/every-biblical-argument-against-being-gay-debunked-biblically?g=2&c=go1
www.acceptingevangelicals.org/
Those Christians who do not think any kind of gay relationship is OK are totally entitled to their opinion too, and I know and love some very traditional evangelical Christians, I also know and love some liberal ones and some like me who are trying to make sense of what the Bible says for these areas and have found themselves feeling differently to committed gay marriage than they expected to feel! 
Most Christians I know (regardless of their views) are very loving and kind in their actions towards others, whatever they personally believe about different lifestyles they behave kindly to all, and this is something I treasure and value. But I really want to be able to explain my views as an accepting evangelical. Any help welcome. Thanks.