Instead of arguing about whether priests are forced to marry gay couples now (they are not, and this is an easily verifiable fact), I think a more interesting question is - what principle is it that says it's ok for priests to deny marriage to same-sex couples?
In other words, can you please explain why priests should not be forced to marry gay couples? "Force" is a loaded word, of course - I'm not necessarily talking about armed agents going into a church and demanding that a priest perform a marriage ceremony, but rather referring to the full range of coercive actions available to the federal government. For example, if a church refuses to perform same-sex marriages, why should they get tax-exempt status?