The Bible has much more to say on divorce and remarriage than it does on homosexuality. Ignoring the vexed question about what exactly the Bible does say about homosexuality (and the assumption implicit in the idea of “what the Bible says“, namely that the Bible is a unitary document which is to be read in the way evangelicals do), the New Testament statements on divorce are clearly and directly against both divorce and marrying a divorcee. Jesus describes the latter as adultery in all of the synoptic gospels. St Paul explicitly states that divorcees must not remarry.
There are a couple of exceptions to the rule: sexual malpractice of some kind (the Greek word which the New International Version translates as “marital unfaithfulness” here, and “sexual immorality” elsewhere, usually rendered “fornication” in the King James Version) and the case where a Christian has an unbelieving spouse and that spouse deserts the believer.
Nevertheless, my impression is that evangelical churches are more willing to re-marry divorcees (whether or not they would be subject to the exceptions mentioned above) than, say, Catholics are, while at the same time being steadfastly against gay marriage. I’ve been asking an evangelical about this on uk.religion.christian, after he made a statement which seemed to confirm my impression. He’s said some good things about repentance and forgiveness in regard to divorce, but hasn’t yet addressed the point that the second marriage itself is described as sinful by the Bible, so it’s hard to see how one can repent of a sin while one is doing it.
I think these churches are doing the right thing in letting compassion overrule “what the Bible says”, of course, but once you’ve done that, why not do it for the gays too? The reason why they don’t do that is, I cynically suspect, because they know what their members want: there are a lot more straight divorcees than there are gay people wanting to get married. As St Jack of Lewis pointed out, it’s very easy to condemn a sin to which you feel no particular temptation.