Every time a church has been challenged for an unwillingness to allow their space to be used for a wedding against their beliefs (including many churches who will not marry previously divorced people, so not just gay weddings), it has failed. The only way to really legally attack a church is through services that a number of churches now consider part of their income, which include wedding catering, wedding music, wedding video/photography, and wedding planning services, which do not HAVE to be performed in the church. Denying someone those services has been successfully challenged, but it also only has about a 45% rate of ruling against the church in the cases I studied a couple years ago for a piece for seminary on the issue. I had a Google Alerts set up for any similar cases that I never shut off (because the topic interests me), and if anything, the rate that a church loses such litigation is actually getting lower as more states are more tolerant and there are more avenues for people to find for wedding services rather than intentionally going after the churches. Basically, the "oh my gosh, my church could get sued" issue has easily definable ways to protect the church (basically, if any church "service" like I mentioned earlier is written into the church constitution, and that constitution has language stating the church has every right to refuse based on belief - which most churches have done already - the church is legally covered), but people often use that excuse as a reason not to support homosexual marriage in my experience. It's rarely about what would happen to the church as much as it is a personal feeling against gay marriage that they don't want to come off as a bad person by stating that feeling, so they lean on something that really has been shown not to exist like their church facing a lawsuit. PR issues that would come from such a lawsuit are another issue altogether, but that doesn't change the reality of what has come of legal court cases that have happened already, providing a pretty well-defined legal precedent.