Heretic

.
The issue at heart is that, in certain forms of christianity, the notion is that sex should
a) only take place after it's been blessed by the church (i.e. only within marriage) and
b) only be for the purposes of procreation - and therefore sex for pleasure is a 'misuse' of a gift from God and therefore Bad. This applies equally to heterosexuals BTW. It's also why the Catholic church is dogmatically opposed to contraception, as it sees it as encouraging sex without procreation, which is Bad.
As homosexual couples can't procreate and can't get married, gay sex is Bad and therefore homosexuality is Bad, because who you sleep with is kind of what defines whether somebody is gay or straight - to be honest I do find it hard to argue with them on that last point

..
Now, this is all complete tosh of course and not something held by any other religion (ask a rabbi if he thinks sex is only about procreation for example) and, in fact, it's not even a view held by some progressive christian doctrines - I suspect the CofE would happily allow gay couples to be married in it's churches. There are quite a few 'out' CofE priests who would be quite happy to do the job, I'd imagine. But it's one that does exist in a lot of more traditional christian groups.
Jon