Actually, FX doesn't say which episode is airing first, so this one may not air tonight, but the first episode among the screeners sent is "Mac Fights Gay Marriage," which isn't quite as potent as viewers might expect from "Sunny," which often takes a topic and finds a new twisted way of looking at it.
In this episode, Mac (Rob McElhenney) only fights gay marriage after he discovers a male-to-female transexual he had sex with is now married to another man, post transition surgery. So the episode is less about an interesting take on gay marriage and more about Mac's jealousy and also the fact that he slept with the woman when she still had male parts, making him more gay than the guy she's married to now that she no longer has male parts.
The episode also runs afoul of reality when two other male characters decide to get married: Pennsylvania does not allow gay couples to marry, nor are there provisions for registered domestic partnerships on a statewide level, although some cities make allowances for that in a way that's more lip service than substantive.
Another upcoming episode, "Who Got Dee Pregnant?" deals with the ramifications of actress Kaitlin Olson's real-life pregnancy when the guys believe one of them may be Dee's baby daddy, including, possibly, her brother, Dennis (Glenn Howerton). This episode, with scenes told from multiple perspectives (some of the small differences are the funniest aspects of the episode), is a much stronger outing for the "Sunny" gang and feels more rooted in their selfish, awful characters.